We've all been there, strugling to figure out what subject we should make our new wiki on at stupid hours in the morning, the rules you should have, and so on, so, here are Kawaii's key tips to making your wiki successful and popular.
Make sure the subject will atract a lot of people, and it is interesting.
Make the subject relevant to what people are interested in, in your country and generation.
Make sure it is a subject the whole community can contribute to, not just a few people.
Make sure the subject is not going to cause a lot of controversy or upset people.
Make the subject applicible to the age group you are targetting. (Example: If you're targeting 13-16 year olds, do subjects like pop culture, or trending movies and books.)
Don't make the entire wiki about one joke you had between you and your best mate.
Don't say the wiki is for one purpose then change it half way through editing.
A few good examples of wiki's that have had success in this are Community Central, a place dedicated to helping users like me and you, Steven Universe wiki, and The Hunger Games wiki.
Have a ground zero for basic rules and expectations of your community.
Have a good set of rules, but don't go over the top and become a dictatorship. Allow your community the freedom in the sense that they can edit in moderation without being in fear of being banned. Make sure the rules are fair and apply to everyone. Make sure your staff know this too. Without communication with your staff, and the staff communicating with each other, your wiki will not work out.
Above all, make sure your Wikia is fun and enjoyable for everyone.
Let's face it, nobody wants to be apart of a Wiki that isn't welcoming and engaging. If it's not, you're gonna have a bad time.