Okay, where to start? I realize that you've just got your camera and you've done a good job considering your experience level.
That said, yes, they are a bit underexposed. That can be fixed by photoshop (or whatever you're using to process your images).
You need to get closer. If all you have is a prime focus lens, get up closer to the subject. I'm sure you can find a spot where you'll be out of the way, but watch out for foul balls!
Try to get away from positioning your subject in the center of the frame vertically. You've got a lot of cut-off legs. Move the head up towards the top of the frame.
Learn how to adjust the shutter speed and apeture on your camera. Learn how to take a light reading and manually adjust your exposure so that your photos won't be affected by a overly dark (or light) background. In sports photography, your goal is the action, not the overall photo (mostly). Adjust exposure to that action.
Also, not sure you were wanting to focus on the catcher in photo #3. If not, your autofocus picked him up and decided he was the primary subject. This is because in matrix autofocus, your camera decides on the largest subject in the frame. Learn how your camera uses single-point autofocus or work with manual focus. If you did want to focus on him, you've got framing problems. He's too close to the edge and too low in the frame.
Hope this helps. Good Luck!