Contact your local MLB affiliate to see if there are any batboy openings or if they're taking applications. It's best to contact the team during the off-season, either by phone or email, since most teams hire batboys before the season begins.
Secure an inside connection if you can, as the waiting list may be long. It helps to be related to someone who works for the MLB team either on the field as a player or in the front-office staff.
Reach a certain age. Again, you need to contact your MLB team to see what their age requirements are. Some teams require you to be 14 years old, others say 18.
Get yourself into shape. You need to be physically able to do all the running around, such as grabbing discarded bats and rounding up baseballs, to become a batboy.
Be prepared to spend many hours with the MLB once you become a batboy. The job often requires you to be at the ballpark two to three hours before the game begins and stay for a few hours after the last pitch. It also doesn't pay well, but the perks of interacting with your favorite players up-close and personal make it more than worthwhile.