Why cant i purchase or install anything on my windows phone?

I have a Samsung focus, from at%26amp;t, and windows phones connect to Xbox Live. I've been trying to install YouTube and games and it keeps saying I have to create a Microsoft account when I thought I already had one? Can someone give me a step by step of how to set one up or what I should do or if anything can be done? Please and thank you :)|||go online on the zune site, and login, and theres some things u have to do, i forgot, but when i got the phone, it said that too, i made an account, it still didnt work, so i logged on the account i made on the computer, and registered it, and now it works, sorry if i confused you, but yeah :)