I'm 19 and I don't know how to conversate with people I just met. I have a very large group of friends and I'm very talkative around them, but when I'm talking to someone new I feel like I have nothing to say to them. I ask simple questions and try to make small talk but it all just seems to end very shortly. When I talk to girls I get nervous and start talking out of my ass and I usually say something stupid. At parties I definately feel stuck and I feel like a loser or something.

You have trouble to talk with unknown people because you lack a common context from which to drawn talk subjects.

The best way to proceed is to ask some exploratory questions to know what are people's interests, and once you find one that motivates you dig deeper into it. If you like some type of movies, and find out that the other person too, then you have something to talk about.

You could try listening to what they say. For instance, someone you're talking to might say, "We had a great vacation! We went skiing and snowboarding with friends!" This presents you with three things to talk about:

1. Their vacation. You could ask how long the vacation was, or you could ask what motivated them to go to the mountains (as opposed to the beach or elsewhere), or something else.
2. Skiing. Ever tried it? If not, you could ask what skiing is like or how you use those pole thingies anyway.
3. Snowboarding. Same as above, but you could ask whether he prefers skiing or snowboarding.

Assuming that you didn't know a thing about skiing or snowboarding, this already presents many options to continue the conversation. You can get conversations going like this easily if you use this method.