Hello I don't have all the answers. Here's the answers to most of your questions:
Does the GoPhone plan include a simcard that i can use in my HTC desire?
I'm not sure if the SIM card would be free if you bought refills from the same at&t store. You would need to have your phone carrier unlocked before inserting a US SIM card, provided that you could legally unlock your phone in your country.
Where can i buy the GoPhone plan?
When you arrive in the US, you can buy a GoPhone plan at an AT&T store. While you could buy a refill card from other types of retailers, they may not have a SIM card, nor support.
Is it possible use more money than your bought the GoPhone with? (im afraid i will get a large bill when i come home
I don't know the answer. One thing I know that is very important is to change your voicemail PIN number ASAP.
Will there be any problem in buying the GoPhone plan when im not a USA citizen?
Tourists and visitors can buy a GoPhone plan. You should be fine. You will not be able to activate your GoPhone while you are still outside of the USA.
You can add additional funds to your account simply by purchasing a refill card from many retailers here in the US.
With GoPhone (prepaid) service you can't spend more than what you have in your account - you won't receive access to the network once your funds are depleted; so you can't go home to a large phone bill.
IMO, the easiest thing to do is to visit a local Corporate at&t store once you arrive in the US - they will be able to fix you up with a GoPhone plan/account/SIM. And if you'll add funds to your account while in the store, they likely will not charge you anything for the SIM.
=^..^= There must be a happy medium somewhere between being totally informed and blissfully unaware.