Stretch marks usually come from extreme situations, such as pregnancy or sudden weight gain or weight loss, and growth spurts, or in rare occassions are associated with some diseases such as Cushings. Stretch marks are tears in your second layer of skin, it just means that there really wasn't enough elasticity to support the sudden change. I have hundreds of them, and its very depressing and hard to deal with sometimes. Many people will give different views on what to do, alot will just plain out tell you that its life and that they'll be there forever.
If you haven't really experienced any of those things, or if you had either way, you should start taking vitamins for your skin such as Zinc, Vit E, Aloe Vera, Vit C, and Vit K, there are also multi-vits that are just for the skin. Start drinking plenty of water to make sure your skin is getting hydrated. Get a good skin lotion such as St. Ives Collegen/Elasticity and apply that twice a day to your skin. I also do an exfolliation scrub using St. Ives Aprocot Scrub, I take one of those wooden massage tools with the rubber heads on them and in fast circular movements I massage the scrubs into my stretch marks, then I take dermabrasion gloves and massage them, then I rinse and apply the creams. I also went out and purchased some creams directly for stretch marks, they seem to help in making them lighter and less ridged. Also its a good idea to eat foods rich in protein and amino acids since they are the building blocks to collegen. And you should do toning exercises to tone and tighten them. Or you can do what I"m doing and get a few tattoos to hide the appearance of some of them but that might be to extreme for some people.
You really have nothing to lose, in the end you'll at least be taking better care of your skin and body overall. They will though eventually fade and grow smaller in time, I think it really all depends on the individual and being very determined. Good luck and don't worry, almost everyone has stretch marks.