Freerolls are a hard way to learn because people don't play with much of a brain during them half the time. So what tendencies you are supposed to see in player simply won't work early in the tournament since they aren't playing with any logical gameplan. Later in it it gets better though.
First thing to pay attention to? If you are playing cash games I would think it would be the general hand strength needed to stack off with (ie don't stack off with top pair top kicker unless you are against a bluff-o-matic, and be careful of board textures). In MTTs, the first thing I'd learn to do is learn to feel when the blinds become worthwhile to attack and when doing so is profitable. Also how to go into fold-shove mode as a short stack
I haven't read any poker books
so I don't know how well they cover things. Assuming you have a decent base to work from, probably good to read some hand histories from here (cash or MTT, whichever one you play). I can't recommend them enough, they are invaluable tools.
Other than that, play some and see how you tend to lose. Then you will know what you need to pay attention to