I have a few questions about when my metabolism will slow down, I'm 17 right now?

You shouldn't have to worry about it significantly changing until you are over 30...however, common sense should tell you that if you notice you are gaining weight, you should adjust your eating and exercise to cause that to stop...you won't just wake up one day and have gained a ton of weight, you'll start to see it gradually and can get it in check well before it gets out of hand IF you choose to do so...