What happens when I buy a phone/ Iphone and what is the best deal?

Im looking to buy a new iPhone 6s Plus and I have now idea how contracts work or what is the best deal. I am 17 years old and I haven't had a phone with a contract in my life. Right now I have a Samsung Galaxy Stardust trac phone that my mom got me but I absolutley hate it because it's so slow and has the worst picture quality. Im not new to apple products since I owned a few iPod touches in the past. So basically my question is: What carrier should I go through and what can I expect when I go to the store to purchase the phone and what is the best deal. Thank you!

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