First you need to determine whether you live in a state that offers online sports betting which you can do
here.
If you do, and multiple options exist, then I would suggest you sign up for all of them and take advantage of whatever first deposit bonuses are offered. It may be best to do this gradually, rather than all at the same time, so as to take full advantage of whatever bonus is offered at each sportsbook. These first deposit bonuses usually come in the form of free bets or "no sweat bets" (if your first bet loses you receive the bet back in the form of a bonus bet which must be wagered).
Be sure to read the terms and conditions of everything you sign up for, so you understand without a shadow of a doubt what it is you are signing up for and receiving, if you need clarification contact support instead of assuming anything. This is where new sports bettors make the biggest mistakes in my experience, so be careful, and of course
only play within your means.
The MMA offerings for each sportsbook vary state to state, it depends on what types of bets were approved by your state legislature, so be sure to look around in each one for the bets that appeal most to you.
Good Luck.