Hello TMM-
Many people around here feel that the best pistol for a person to learn on is a .22. They have virtually no recoil, not too loud and you can shoot all day for practically nothing. They help to ingrain some good habits and spare you some difficulties in starting with a full-on combat caliber.
If your tagline is right and you are still 15 years old, then you have either 3 or 6 years until you can apply for CCW, depending on location. I would suggest starting out now with a .22, if this is the case, and then purchasing a Glock or GP when time to carry is nearer.
If this isn't the case and you're already close to application time, then I would pick what you're most comfortable with. Which feels better to you? If you don't think you'll ever carry the GP, you should probably stick with the Glock. Though you could get one of the SP-101's, a smaller revolver, which carry quite nicely.
A plus for the revolver is that it can be loaded with very light .38 special loads which would be much better to learn with. Then full-power .357's can be used for carry.
Good luck in the decision!
Editted to add: I forgot the make note that the most important choice you can make, much more so than which type of pistol, is to get some training! Something like the NRA basic hangun course would be great, something to teach you the in's and out's of things like safety, technique, maintenance, etc.. Others can probably advise you much better on this.
