The general idea is this:
Each fantasy pool has a different scoring system. The scoring system is so important because some players perform better in one system than another. For example, a pool might offer points points for every 15 yards gained/passed. In this type of scoring system you might want to draft the best QB over the best RB simply because most QB's will rack up more yards. The idea is that after you understand the scoring system, you draft the players that you think will perform the best in that particular pool. Most pools takes turns between the owners during drafting, and no player can be drafted twice. The pools generally start week 1, so the draft has to be done by then. Also the pool ends about week 15, so there can be a week 16 playoff. The reason it is week 16, and not 17 is because alot of teams sit their players on week 17.
In some pools you start different players each week, in others all of the players on your team accumulate points. What 'starting' a player means is say you have 3 different QB's and your league requires you start one every week, then you would probably start the one with the best matchup. Like Kurt Warner playing against New Orleans. Therefore only one of your QB's that week would accumulate points. I prefer a pool where you start every player, every week. It's just easier that way.
There are many different types of league, some include defense, some subtract points for interceptions thrown/fumbles, in some leagues you keep your players all the way through next year as well, some pools you can drop/pickup guys throughout the year. It just becomes quite complicated. It all depends on the type of league you are interested in.
Like I said though, the key is understanding the scoring system. If you plan to join a pool with your friends who know the system very well, it is likely that they will have a strong advantage over you.