Sports photography is a very demanding form of photography on the photographer and the gear. Lets start with gear. If you are shooting in good light, ie. mid-late morning or early-mid afternoon then you choice of lenses will not be quite as important as you should have enough light to get the shutter speeds you need. If you are going to be shooting in late afternoon-early evening or under the lights then at least an f2.8 lens is a must.
You need a minimum of 1/320th second for baseball and 1/500th to 1/1000th would be better. If you do not feel comfortable shooting in manual then shoot in Aperture Value. I know some people suggest using shutter value however you can accomplish you goal just as easily with potentially better shots in AV mode. The difference between a good nice sports shot and a great sports shot is not just capturing the action. It is also about making that action pop out of the picture. You accomplish that with using just the DOF necessary to have the action in focus and not the surrounding players, field, stands etc. Set you body on AV mode and select the aperture you want to use. I generally am in the f2.8 range up to f4 for outdoor sports depending on the sport and where I am shooting from. Now, control your shutter speed with your ISO. Set your ISO just high enough to get that 1/500th of a second shutter speed or faster. When shooting in daylight that ISO is not going to go up much if at all from 100 or 200. In AV mode it is easier to keep that shallow DOF you want on the fly and quickly adjust your shutter speed with your ISO than it is to have a constant shutter speed and try to get the DOF you want.
Select a single focus point and set you body to full time focus. Canon calls it AI-Servo. Don't know what Nikon calls it. If possible get your the focus off of the shutter button and onto a button on the back under your right thumb. Use the shutter button only to trip the shutter. Keep the camera glued to your face and keep focusing on the action at all time with that single focus point. Anticipate the action. When you are shooting sports those kids on your team are not your friends, they are not your classmates and they are not people you care about or want to talk to. They and the opposing team are potential action. Also the only thing you care about in the game is the action. Doesn't matter who wins or looses. If you get wrapped up in the game you will miss shots. If you get wrapped up in only the action you will be prepared to capture the action. You can celebrate or feel bad with them after the game.
Know the game and know the players. That means not only the rules, but the psychology of the game. Baseball is my favorite sport to shoot, as I caught for 11 years. I understand the game from the inside and it gives me the ability to really anticipate the action. Know the players on the team. Who is a great defense man and who is a hitter. It makes it easier to anticipate that ball contact with the hitters and the great plays in the field from the great defensive players. Remember though lousy hitters get hits and lousy gloves make catches so pay attention when players are not at their strength.
Shoot tight enough to get action, but not so tight that you cut of arms or heads. If you must cut off legs try to do so from the waist in most cases. it is better to not cut off body parts. You can always crop later. Finally, the difference between a really great sports shot and a crowd stopper is between the shoulders and the hair line. Get the face and get the expression. That face and expression is what lifts any sports shot up. Be it the thrill of victory or the agony of defeat. Sorry, I couldn't resist that one.
Last but not least. Practice. The players do and so should you. Go to some practice sessions and shoot. Practice your technique. It will make the basics easier to do on the fly allowing you to concentrate on catching the action. Good luck and have fun.