I'm on the shy side, and I've had to learn how to manage.
So the first step is harder for you than it may be for others. You'll have to smile, say hello, and introduce yourself. It's not your job to make them like you; it's just your job to speak up. Think of questions in advance to ask these other moms: Which one is your player? How long has your child been playing in this league? How does he like it? Do you have other baseball-playing children? Ask them questions about themselves, and they'll probably be delighted to answer. People *love* to talk about themselves! If they don't ask you questions in return, that's all right. Just listen to what they say; keep being genuinely interested in them.
All that said, it doesn't mean you'll become bosom buddies with these moms. If you find one who is pretty friendly, you're doing well. But don't try to "make" them like you - you know what I mean - and don't worry what they think of you. Just speak up. Oh, and if you can volunteer to help in some way, do it. Getting more involved makes you feel more a part of the group.
Remember, it takes lots of time to make a *close* friend. All you need to do right now is make acquaintances.