how to get an at&t plan when i travel to US?

They might do a prepaid plan where you buy some minutes but AT&T does not do 20 day "plans".

 

I would think your best bet would be a prepaid plan from a 3rd party provider (MVNO) that uses AT&T's network. Try AIO wireless or NET10 wireless.

 

I would also consider looking into Straight Talk or possibly even T-Mobile. I wouldn't even bother with AT&T.

 

I've done this a couple times while going to the U.S. with an unlocked iphone.

Go to AT&T and get a prepaid plan. I think I paid around $50 for 1GB of data and unlimited calls for 1 month. I'm sure there are better deals, but I just wanted to spend as little time as possible getting my phone on a local network.

 

That's correct, you won't get the best deal, just for the $^%#& SIM alone is $25 bux, but AT&T stores are everywhere for conveniency sake.