Depends if you want to build a webpage or not.
If you just want to forward it to a casino site, there
are lots of good affiliate programs to choose from.
Stick with either a poker site or a slots site, those two
account for most casino revenues.
If you're going to build a webpage or site, then you can
make more of a directory, listing many casino, bingo, or poker
sites and a short review of each. You could also sell gambling related material, like 'how to play poker' ebooks, etc. I prefer this way,
because then YOU are in control of what goes on your page, and
you're not locked into one affiliate program.
good luck