Go to the ITF or USPTA website, find a Futures tournament in your area or close to it. Sign up and hope not enough higher ranked players show up and get into the qualifying. Simple as that.
....or play some prize money/open tournaments in your area first, get a half decent ranking at least to increase your chances of getting into a Futures qualie.
....or take $5000, go to Eastern Europe and buy yourself a wild card into a Challenger.
Thats it. Any knucklehead can get into a Futures tournament, ranking is not required, it depends on who shows up. If they don't have enough players for qualifying they'll take anyone who can pay the entry fee, if its packed well thats a different story of course. Most of the guys playing Futures will be playing open tournaments for money, so that would be a good guide of where you want to be playing wise. Good luck.