you should check with your service provider that your account is enabled for international roaming before you go, and be aware that firstly the mobile coverage in America is a little sketchy when you get out of the large cities and also watch out for the charges! you will be charged extra for not only making calls and sending messages, you will also get charged to your account for receiving them.
ok you can just dial 4444 for free to top up while aboard if you need to and use a debit card to top-up
it will cost you 99p a minute to receive a call while in the Us and �1.49 a minute to make a call (to a mobile... 99p a minute to a uk landline) also it will cost you 49p for every text message you send or receive.
so as you can see it can work out pretty expensive if your not careful!