Extremely dedicated advertising is a strong part of it, as is keeping the few people you end up with starting out and letting them do some of the work for you. Beyond that, my main tip is to not try and be a clone - for example, you will probably flop quickly if you try to make a site that is heavily based off of Planet Minecraft. Inspiration is fine, trying to come to the same end is not quite as fine for success chance.

Probably by getting a popular Youtube channel to promote it or by paying for advertising. After you get even a small user base, word of mouth does a lot of work for you if the site is decent.