This may have been covered already:
Fantasy Football is a game where your team is rewarded points for the statistics they wrack up during the actual games on Sundays & Monday nights (see the league settings for actual stat categories and points). The most important part of the entire season is draft day, which the actual players you draft that day to form your teams will be yours alone for the entire season. You are stuck with those players and nobody else can have them unless of course you decide to trade or drop them for free agency.
For the draft you should be sure to check out draft results, rankings, and depth charts to aid your decisions during the live draft (there are many free resources out there to help you). You can make the decisions yourself or yahoo will draft for you based upon their pre-generated rankings. Yahoo is for the most part a pretty good estimation of expected performance, however, there are alot of steals out there that are undervalued by yahoo's standards.
During the season you will need to set your starting team each week so watch out for who on your team is injured or switch your roster for favorible matchups. You will have enough slots for backups for each position to rotate for offweeks, injuries, or poor performance so keep that in mind while drafting as well. I guess thats a good start. Some people invest several hours in research to get the best possible team while others flip a coin and let yahoo figure it out, both ways can turn out good or bad.
A couple pointers from my past experience in all fantasy sports:
Don't draft players because you like them or the team that they are on - draft the best possible player at that opportunity because you will end up with the best team
Try to draft the fewest amount of players from the same team as possible - this will give you more balance so you won't take as big a drop on off weeks and if that team sucks during that game or goes up against another good team your team won't completely die.
Don't draft players that are good on field: ex. M Vick and D Staley last year players that either played very well and helped their team or were regarded on great players but didn't help their respected fantasy teams.
Remember it is all about stats!