It depends on where you live....... eg... Texas is real big on football, and there are rec and semi-pro teams in all the larger cities.
Here in dallas there are several semi-pro teams (I play on one myself) also 8-man tackle, 6-man tackle, ruff-touch and flag teams for adults.
I suggest that you start with flag-football. That way, you can get a feel for what positions you are good at. Also, you won't have to know how to tackle,..... just have to be quick enough to get the flags. When you get confortable with a position, then you can look into tackle teams.