Another Window

I have a '93 gmc 1500. The passenger window doesn't go up on it's own. I've heard it's just an adjustment but I can't figure out how to adjust it. It goes a few inches up, and then the front end of the window will stop, the back will keep going a little bit, it'll get crooked and then stop completely. The only way to get it up is to put upward pressure on the font of the window. Any idea will help, thanks.
Steve