You see many go postal early on. Nothing worse than a luck box shoving with total garbage and sucking out. Early on, it's best (IMHO) to play post flop - you really need to see that flop before going to war.
What you need to do is play a solid fundamental game early in a freeroll. The mistakes made by the fish early on are all classic ones. Realize that, and you can use it against them;
1. Too many hands played.
2. Limp in UTG and then call raises OOP with God only knows what.
3. Donkey betting (betting post flop OOP into the original raiser)
4. Limp in with big hands vs raise with them.
5. Betting tells
; they raise pre flop with big pairs huge (4x-8x), small pairs less (3x-4x), and limp/call Ax/Kx etc.
6. Kicker trouble (kicker? Whats a kicker?); plays weak aces, kings, you name it.
7. Bet at obvious boards, making their hand obvious (sets, flush/flush draw, etc)
8. Bluffing calling stations. This is a big one. It's how weak players get big stacks early on.
9. Chasing draws without the proper pot odds
to do so (pot odds? what are pot odds
10. Calling/betting dumb end of st8's and weak as hell 4-flushes.
11. C-betting weak, allowing draws to make it.
12. Betting st8's into flushes.
13. Betting flushes into full houses
14. Not letting go of one crappy pair on a dangerous board or to an obvious overpair
15. Calling shoves with ANY pair.
16. Stubborn raise calls from sb and bb (you are oop post flop) with horrible hands (see #6)
I could go on, but you get it.