"I only have normal sneakers...Is that OK ?"
Yes. I wear ones like these, but the soles have worn down so much they're practically flat. I should buy some new ones. Normal trainers are fine.
"Anything else I need to know?"
No. Just come down and have a kickabout. I might make a little graphic about the rules.
The main thing is to try to have fun. That can be as simple as playing a few short passes now and then, not hacking the ankles of the organiser, and not smacking the ball as hard as you can at the female goalkeeper from four yards away.