Basic baseball leagues are usually Roto 5x5 (5 hitting categories, such as AVG, Runs, RBIs, HRs, and SB, and 5 pitching categories such as ERA, Wins, Saves, WHIP, and Strikeouts). Some leagues play for total points based on these categories, where you try to finish as high as possible in each category. Some leagues play Head to Head for a week playing for these categories. Example, if you win 7 categories to 3, your overall record is 7 wins and 3 losses. There are other formats as well. Some leagues are salary leagues, where dollar values are assigned to each player, with usually a maximum cap of $260. Other leagues assign contract years to players. Personally, I only play in one league. It's a dynasty league with no salaries or contracts.
A good resource for Fantasy Baseball are
Rotojunkie.com,
Fantasy Baseball Cafe,
The Inside Pitch (good for closer information),
The Scouting Book (for prospects),
Fantasy Baseball Hub (has lots of links for FB),
Melnik & Greco (good podcasts),
Sports Fanatics (Decent for rankings)That's a few links to get you started. Good luck.