I have ten dollars on my amazon account and I don’t know if I can buy books for my kindle or not. Can I? If so, how?

My parents ordered me a kindle from amazon for my Christmas present this year, but they didn’t know that unless you mark that it shouldnt be registered, that it will come registered. So, is there any way that when I get it I will be able to deregister it from their account and register it on to my Amazon account? Thank you!

Okay I got a kindle 2 for a gift and it came automatically set to the persons amazon account who gave it to me. How can i change this to my own?

I want to give my original kindle to my sister and buy myself the new edition. But, I also want her to be able to read all the books I have paid for and loaded already on the original kindle. Ideally I would deregister the original kindle before I give it to her so she can re-register to her own amazon account. But then will my old content disappear?