How can I get motivated to lose weight? (exercise and diet... )

Well it depends on how old you are.
When you are older you want to take better care of yourself, you have to. Think about the cost of healthcare.
When you're younger, I don't know look at yourself stark naked in the mirror every day. If you don't like what you see, get off your butt and push yourself away from the table.
But no matter how old you are, you will just feel better physically and emotionally.

But until you are really ready, there is no pat answer

I'm seventeen and I am positive that ready to lose weight, I just don't know how to get started. :(

Burn more calories than you take in. That is the key. Eat right and exercise!

You're not going to lose 20 pounds in a month but you will lose healthily