Grade point average; it's basically an average of your grades in high school or college. Most are on a scale of 0 to 4.0, but it is possible to be higher depending on the particular grading system.
Liberator, regarding your current mental state, I kind of understand where you're coming from. I'm a college freshman who has never had a real job, has never had any girl be interested in him, is a self-proclaimed anti-socialite and introvert, and doesn't have any real friends. You know what? I'm happy with who I am.

There are some things about yourself that you just can't change; you just have to accept them. Regarding myself, I feel uncomfortable talking to almost anyone outside of my family; this probably explains the no friends/no girlfriend situation. This doesn't mean I won't ever make some good friends or find a true love; it just means that I haven't up until this point. I know that I've gotten a lot better and have opened up much more since my earlier years; it's a continual process of growth, and I'm still working on it.
As for yourself, what I think you need to do is first to talk to someone, whether it's some sort of psychiatrist, a clergyman, an adult that you trust, a relative, or even your parents. Just talk about what your situation is and why you feel hopeless about it. Just doing that much will make you feel a little better. After that, start to look at your problems one by one. Regarding your GPA, do you have problems understanding material, completing the work on time, taking tests, or something else? Whatever it is, just pick one area that you need improvement in and start to think of ways to improve it. (I'm not sure where you are in the educational process, so I really can't be more specific.) Regarding the job/money situation, why exactly were you rejected. Were the places you were applying not looking for anyone, were the managers just douche bags, or do you think that it was something about the way you came across? If you think it was something you did, try to work on your approach/demeanor. If it was the job, all I can say is keep trying. (I'm going to have to find a summer job myself, and while I'm dreading the prospect, I want to make it as painless as possible.) About the friends/girlfriend, are you like me in that you have problems talking to people? Have you always been this way, or have you been able to feel more comfortable with people over time? Do you lack confidence in yourself? There's not much I can advise you on here, seeing as how I'm almost in the same boat, but I will say that I truly feel that you will someday meet people who like you for who you are.
Above all else, you need to feel confident in yourself and be comfortable with who you are. This isn't easy to do, but with some prayer and some hard work, I know you have what it takes to improve your life.
A.K.A. Mongoose, for you HLP denizens