Pick a league to play in. Fantasy baseball is most commonly played online through one of various sites that offer customizable formats, many of which are free. However, fantasy baseball leagues can also be set up and operated by a league manager without the use of an online format.
Know what kind of league you are playing in. There are rotisserie leagues in which teams get points based on different categories and the team at the end of the season with the most points wins. There are also head-to-head leagues in which two teams will play each other and the team with the most points wins the match. The team with the most wins at the end of a head-to-head league is the season champion. These are the basics types, but some leagues may have alternative or additional rules including combining head-to-head and rotisserie rules.
Draft your team. Your draft will probably be on a set date so know when it is so you can be there, otherwise you may get virtually drafted and have a team you do not want. There are two typical kinds of drafts in fantasy baseball. The first is the auction system in which teams are allotted a certain amount of money and can bid on players. The second is a serpentine draft system where teams draft players in a determined order until all team rosters are filled.
Find out how scoring works in your league. Typically fantasy baseball work on a 4x4 or 5x5 scoring system based on either 4 or 5 offensive and 4 or 5 pitching statistics. The stats for a 4x4 system are usually team batting average, home runs, RBI and stolen bases for offense and wins, saves, ERA and WHIP for pitching. For 5x5 leagues, runs and strikeouts are often added. Some leagues go to a 6x6 system or more, adding in other statistics like OPS and holds.
Assemble your starting lineup. You will have players at all of your defensive positions, starting pitchers, relief pitchers and utility players. Be sure to make sure the players you are starting are not hurt or in a terrible slump.
Make trade requests and pick up free agents when needed. Since you will usually have your draft at the beginning of the season, it is easy to end up with players who had a good year last year but are terrible on your current team. In many leagues you can trade with no penalty and you can pick up free agents. You can only have a specified number of players on your team, though, so if you are picking up a new player you may have to release on of yours first.
Keep up with your team throughout the season. A fantasy baseball team can run fairly autonomously, but in order to be successful you really need to check your team on a regular basis to see how players are performing and decide if changes need to be made.