Depends on the gun.
The F-class X-ring is 1/2 moa. Ideally you want a rifle/ammo combo that will at least do that.
However, if you are just looking to get started, use what you've got. Don't take this the wrong way, but in your first match, you're not going to be competitive anyway (unless you already have competitive rifle experience). The main thing is to get out there and do it.
Best thing would be to let the match director know you want to try it out. I'd be surprised if they didn't offer you a rifle and ammo to use.