You're supposed to know that before you start the site You will probably need to start at the top of the SEO to do lists and work your way down all of them. You're not going to find any easy method with the niche you selected. I would try and promote to other types of sports enthusiasts and not just gamblers. Hit the hell out of the sports forums and supply a lot of good odds and advice. Write some arbitrage betting articles. Everyone likes the idea of not losing. Colleges are a good source of new sports betting enthusiasts also. You want to nab the recruits while they are young and before someone else gets them. A lot of the male poker playing crowds like to bet on sports also. Best of Luck!
If you know a lot about a particular sport that people can bet on, then you could give betting advice. Ex: You could give weekly betting advice on football games, and explain why people can win by following your advice. It's easy to give advice on something like sports, because no one can tell if you´re good or bad... Another way is to make a sports news site, though the competition here is hard.
I suggest a free sport arbitrage service. You can make easily money. SportSureBet is the best free surebet service: sportsurebet.com