How Google works
Your wikia's Google ranking is based on three things: rich content, use of keywords and high-quality links.
This is the most important part of creating a wikia — adding interesting and relevant content. That's what readers and contributors are coming to the community for, and that's what Google picks up on. The more content you have, the more likely your wikia will show up in search results. Content is the text on your wikia, but also images and multimedia. File pages can be just as important for a community's SEO as the article pages.
Use of keywords
When Google indexes your pages, they take into account where the search phrase is on the page. If a phrase is in the page title, in a big heading, or repeated multiple times on the page, then it will rank higher than another page that has that phrase in small type at the bottom of the page, or has only one occurrence of that phrase.
Part of your ranking is based on how many pages link to your wikia, and how high those pages are ranked. When important websites link to your wikia, that's a very helpful "vote" for you, so it's good to get as many links from popular sites as you can get. The core of a wikia, of course, is all of the links between pages. Those are important to Google too! Create as many pages on your wikia as you can — even stub pages are okay, as long as they link to each other. Category pages are hubs of links on a wikia, and with some quick work those can also become a great part of your SEO strategy.
Step by step
The following tips will help you raise your wiki's Google ranking, but the most important "trick" is just to keep working on your wikia, making it bigger and more interesting. The more Google sees things changing, the more often it will come back to index the latest changes.
Step 1: Name your wikia after the topic
Your page rank is partly based on your wikia's sitename in the title bar at the top of the page. So you should resist the urge to give your wikia a clever name — if you want your wikia to show up when people search for "charles dickens", then "Charles Dickens Wiki" is a better title than "Scroogepedia".
The URL for your wikia (the part of the web address that comes before "wikia.com") is also important to your search ranking, and should be chosen in the same way. Search for your topic's name on Google, and you'll see that the top ten results almost always have that phrase in the site's title, the URL, or both. That's where you want to be!
Your wikia's sitename and URL are set when the wikia is created, and both can be changed in the future by Wikia Support, upon request. Other adjustments to the Pagetitle and Description (Note: The Discription is what usually appears in the search results when someone's search results in a particular Wikia) system messages can further optimize your wikia's appearance in search engine results.
Some communities, such as the Wackishly Awesome Randomness Wiki, are easier to find on Google, because they contain a unique word, which in this case is "wackishly". Other communities, such as the Randomness Wiki, do not have a unique word.
Step 2: Start building content
Before you do anything else to promote your wikia, you have to get down to the business of creating some pages.
Start with 50 pages as your goal. That seems like a lot, but just go ahead and make a bunch of stubs. If it's Charles Dickens Wiki, then throw together a page for each of his books, and start making pages for the characters. Each page can be two sentences and a category tag. If you're fast on the keyboard and you know all the character names, you could throw together 50 stub pages in one evening.
You know your subject, so coming up with those early stub pages is as easy as making a list of names. If it's the Solar System Wiki, then make a page for each of Jupiter's moons — that's 63 pages right off the bat. If you've got a God of War Wiki, then make a page for each of the monsters.
Don't worry about making those pages perfect, or even informative. Just get them started. The only thing you should worry about is putting them in relevant categories, and making sure that a reader (or a Google searchbot) can get from the main page to every page on the wikia just by clicking on links.
Plus, adding all these stub pages will give your contributors something to work on when they show up! A wikia page that's half-finished is always easier to work on than a wikia page that hasn't been created yet.
Building content is your #1 priority. We can't stress that highly enough. Don't move on to the next steps until you have at least 50 pages with 200-400 words each on your wikia.
At this point, you can look for some of the major players in your topic area that allow people to post links.
For a TV show or movie, you can submit your wikia to IMDB. Go to the page about your topic, click on "Miscellaneous links", and then click "Update" to submit your own link. (You'll need to log on as an IMDB user.) It can take a week or two, but the link should show up.
Other good prospects for TV shows are TV.com and TVguide.com, both of which allow people to post links in forum postings. Go to the forum on the show that you're interested in, and post a brief description of your wikia, with a link.
You can also submit your site to the Open Directory Project (dmoz.org). Just search for your topic, select the appropriate category from the list that comes up, and then click "suggest URL". You can submit the URL, and a brief description of your wikia.
There are many other possibilities, depending on the subject matter of your community.
It can also be helpful to have links from other Wikia communities, which you can post yourself. But be careful with these — if the people who are on that wikia think you're being overbearing about it, they'll block you and remove your links. Be cool about it, and only post links that are relevant.
Beyond that, do a search for your topic, and see if there are other fan sites where you can post relevant links. Don't spend too much time on this — you don't want to get distracted from building your site — but it's worth a few minutes.
Step 5: Add more pages; add text to category and file pages
How many pages do you have by this point? See if you can get up to 100. Once you've got a good number of pages on your wikia, you will probably have quite a few categories created as well. Category pages are lists of links that are very helpful to Google, and you can make the pages even more helpful by choosing "Create" from the Edit menu and adding text at the top of the pages. A short description of the category, and the types of pages that are included in it, will make the links on a category page even more beneficial to your site's search ranking.
You can also add text to file pages, to give Google more information about the file and what it's doing on your wikia. Adding a short description of an image to the image's file page, containing key words related to the image content, will allow it to appear more easily in image searches. The same is true for video file pages. While some files, especially videos, may already have descriptions and key words attached, you can always add more information that is specific to your wikia and the file's purpose there.
Step 6: Post on blogs and forums
Do a Google search for your topic, and see what comes up in the top results. Here are efficient key words:
<topic> "submit site" OR "submit a site" OR "submit your site" OR "site submission" OR "submit website" OR "submit a website" OR "submit your website" OR "website submission"
<topic> is the topic of your wiki. If blogs or forums come up near the top, then post comments there, each including a link back to your wikia.
Try to make your comments intelligent and relevant; that will make them more likely to stay up and drive traffic to your wikia.
Step 7: Create pages for other websites
If there are other fan websites on your topic that you admire, create a page about that site. Take a screenshot of the main page for an illustration. Write a few sentences, and post a link to the site.
Once you've done that, write to the webmaster of that site, and send them a link to the page you've created about them. Invite them to come to your wikia and add more to their page. They may end up adding a link to your wikia, which would bring a Google boost and more potential contributors.
Step 8: Don't give up!
From here on, all you have to do is keep adding content to your wikia. You've done everything that you can do to bring attention to your site, so now you can focus on making it bigger and better. By now, if you've got a good title, 200 pages and a bunch of links, you should be moving up the Google listings. And with 200 pages, you may also qualify for a Wikia Spotlight, advertising your project on other active Wikia sites! If more contributors haven't joined you yet, then they'll be coming soon.
So just keep building your community — and when contributors start showing up, make sure you say hello to them, and welcome them to your site. You've put a lot of work into building the wikia. Make sure the new contributors know how much you appreciate their help — see Help:Making a successful wikia for more good ideas!