What are you having trouble with?
The league begins with a draft, just like the real NFL draft, only it snakes (so 12th in the first round gets 1st pick in the second and so on). Different leagues have different rules on how many players at each position, and there's usually at least one flex spot (where you can play a receiver or runnign back). You also draft players to fill your bench, usually players who will replace your starter in case of injury or the starter is on his bye week.
Each week, your team plays against one of the others. You set your lineup before the games start, and each of the players on your team earn points based on their performance (6 points for a td, 1 point for every 10 yards gained etc. Again, this varies league by league), these points contribute to your team's total. After all the players on yours and your opponent's teams have played for the week, the team with the highest total wins.
The league is split into divisions which work the same as the real NFL, the teams with the best winning records at the top and qualifying for playoff spots. Because not every NFL team makes the playoffs and therefore not everyone would have their full compliment of players to field, fantasy playoffs usually take place in week 15 and 16 (again, dependant on league).
That's the basic gist. I won't get too in depth, but you can also offer up trades with other teams and pick up free agents on the waiver wire (Yahoo seems to fuck me every single week with this, so I can't explain exactly how it works in our league, but the basic idea is that the worse a team is the better their waiver priority. So if we both wanted to pick up Jay Cutler for example, but you had a 3-2 record while I was 4-1, you would get him. I think). Once player's team has started its game, your selection is locked, so you need to keep an eye on things like injury reports to make sure you don't put someone in who isn't going to play (similarly, keep an eye on thewaiver wire for back-ups to starters who get injured and will miss time).
Just checked our leaue page. Roster is as follows:
QB
RB
WR
TE
3 x Flex (RB/WR/TE)
K
DEF (You pick a team defense, not individual players)
You score points for the following:
1 point per 25 passing yards
1 point per 10 rushing or receiving yards
1 point per catch
6 points for a passing or receiving TD
7 points for a rushing or return TD (I don't know why, I'm pretty sure Maxx is on crack)
-2 for an INT or lost fumble
3 points for field goals under 40 yards
4 points for field goals between 40 and 50 yards
5 points for field goals over 50 yards
1 point for PAT
-5 for a FG miss under 20 yards
2 points for a sack, INT or fumble recovery by your defense
7 for a defensive TD
2 for a blocked kick
Points Allowed 0 points10
Points Allowed 1-6 points7
Points Allowed 7-13 points4
Points Allowed 14-20 points01
Points Allowed 21-27 points-10
Points Allowed 28-34 points-2-1
Points Allowed 35+ points-4
There's a couple of sites I can recommend for further advice:
sbnation.com/fantasy
nfl.com/fantasy
rotowire.com