Team Fortress 2 is all about playing as a team in a way that's different from other games. If you go out alone trying to get kills, you'll die. This is true even of experienced players because each class has a counter, so it's only by working together to make up for what your class is lacking that you'll be successful. I'd say try some Medic. It's the most obvious class for knowing what to do - point at your teammates and heal them. While you're doing that, you'll notice how others are playing and then you might have some motivation to try other classes.