Im quite new to fishing myself but ive caught tons of flatties,
As long as you're in the right place, a basic flapper rig catches them. The biggest problem with flatties is getting the hook out after you've caught them!
Flounder have fairly small mouths but tend to swallow the hooks quite deep so 2's and 1's will guarantee a hook up but can be hard to get out, so I prefer to use 2/0 kamasan b940's, which are still narrow enough to fit in their mouth but more likely to lip or head hook.
I've found worm baits are best - ragworm or frozen lug - make sure there's no bits of worm dangling about as the flounder can just suck the worm off the hook.
I've also found that plain weights work better if im flattie bashing in the surf on a beach, but grippers work better in the rivers.
And flatties off the beach taste much better than flatties out of the muddy rivers!
Good luck.