If you want a snake that can be in a 15 gallon, I would suggest a Kenyan Sand Boa. I don't have any experience with snakes but I know that the largest Kenyan Sand Boas reach only 2 feet and can be comfortably housed in even a 10 gallon. If you want a smaller snake I would get a male, female snakes tend to get larger than males.
I also know that ball pythons tend to be picky eaters, and require a close watch on humidity.
I really want a corn snake in the next couple of years, I've heard they are great beginner snakes, but they require a larger tank (20-30 gal as adults).
Like I said, I'm not an expert on snakes, but I would suggest a corn snake even though they require a bit bigger enclosure, and then after having that for a while, you can get a ball python or another snake. If you get a corn snake as a baby, they can be in a 10 gallon enclosure until they get a bit bigger to have to be upgraded. In that time, maybe you could save up to get it a bigger enclosure?
I would say do your research on corn snakes, ball pythons, and numerous other snakes and see which one is right for you. You could also look for reptile expos in your area and see those snakes in person to get a better understanding of them.
Usually beginner snake owners get ball pythons or corns, and then later get other snakes like western hognose, Kenyan sand boa, king snake, milk snake, etc.
Hope I could help
