Before I begin, I just want you to know that it's not easy to just pick up a baseball and start learning how to throw different pitches. You need to get your arm in shape first before you go balls out. I would start doing "long toss". Warm up with somebody and gradually make the distance between you guys longer and longer. It'll help build up arm strength. Do that everytime you play catch. If you just go out there, you'll blow your arm...especially if you're trying to throw sliders and stuff.
As for the pitches:
Fastball - You want to grip the ball so that your index and middle finger (keep them slightly apart) go across the "horseshoe" part of the seams, which should look like < or > if you face the ball towards you. Hold it with a light grip...leave some room between your palm and the ball. You'll get the most velocity out of a pitch thrown like this. Keep practicing to get your control down!
Slider - Be careful with this pitch!! You'll tear up your elbow if you don't know what you're doing. What I do is hold my index and middle finger together...or slightly apart. I place them inside the || part of the baseball where the 2 seams almost come together. Now, when you go to throw it..right before you release it, you need to twist hand to the left (if you're throwing right handed). It's almost like a curveball, but you're releasing in the opposite direction. It's a very unnatural way for your arm to twist and can lead to many problems. If done right, your ball should tail down to the right and drop (if throwing right handed). Don't expect to master this pitch at all.
Forkball - You put your 2 fingers in the same position as a slider, but this time spread them out so that they rest on outside of the 2 seams. Spread your fingers out until about 1/4 of the ball is above the 2 fingers. Throw it like a normal fastball, but if done right...then you should get a little sinking action on your pitch. Also a tough one to learn and master.
Hope this helps you a little. I'm just too lazy and tired to find any sites right now , but I know there's plenty. Email me sometime...
tacos@pacbell.net and I'll help you with anything you need for baseball.