Best thing for beginners is to learn all your moves; go through exercise mode and memorize everything. It's easier once you realize most of the moves are just variations on certain strings, like PPP, PP6P, PPK, etc.
Watch match vids and/or consider buying the strat guide (although the next big game patch may render this obsolete).
Read the character forums and find out what your character's best moves, setups, and crushes. If there aren't any listed, ask!
Punishment in this game is whatever, honestly. You punish in DOA with throws, but they don't do that much damage unless you catch someone trying to hold. -8 is the magic number that everybody can throw punish at (-7 for grapplers). You can forget about properly throw punishing online, since everything is safe due to lag.