So, I'm going to be a professional baseball player. I've never swung a bat at a baseball or softball before in my life. I went to buy one and there were so many options. Some were marked "baseball" and others marked "soft ball" some were fat, some were skinny. Some were short and some were long. Some weighed more and others weighed less. What's the difference? How do these factors affect batting? Thanks.

A baseball bat has a smaller "sweet spot" is usually shorter and lighter.

A softball bat normally has a longer barrel (Where you hit the ball) and can be a bit heavier.

Weight is a factor in how you swing, a heavier bat is slower to pull around, but stays on plane, a lighter bat rips around quicker but you can lose transfered energy to the ball.
The length is personal preference, some people like to crowd the plate, so they like a shorter bat.
Fat and skinny, in reality doesnt matter much, the sweet spot is still the same size, ideally you want a skinny bat with a lot of wiggle room to foul back.
Can I get an autograph?

softball: long, skinny, heavy, 34''

Baseball: Depends, Depends, Depends, 28- 33''

With softball bats, the barrels are the same length, wheras you might have noticed baseball bats get larger at the end. Short and long is a matter of preferance, based on how far you stand from the plate. I personally have 2 bats, a light one for fast pitchers, so I can swing it faster, and a heavier one for slower pitchers, because they pop up if you use too light of a bat.

softball bats are usually lighter .. my bat is 23 ounces and that's pretty heavy compared to the rest of the teams .. also a baseball bat has a skinnier barrel .. while a softball sweet spot is bigger