I'd suggest looking through the tags for ad layouts here in this community to get an idea of what you want. Be sure that your ad really demonstrates what your community is about and is eye-catching without being annoying or overdone, which is why the templates here are a good resource. Try this tutorial for making sure your color schemes work.
I would also suggest making sure your grammar and spelling are up to par, because people will judge the game based on how the ad is set up. If they see clashing colors, poor spelling or grammar, and an unclear premise, they'll be less likely to join.
Come up with an ad idea, and post it to critiqued after you have set up your game communities etc. The lovely people there will help you make corrections and give suggestions that will make your game as appealing to as many people as possible. If you're not good at html, I'd suggest finding a co-mod to help you with that, and for your spelling and grammar as well. You can try posting for one at modcooperative.