The best plan is to order ahead of time, and arrive with a US SIM card in hand. Otherwise, T-Mobile have stores in many locations throughout the US, and provide an excellent $30/month prepaid plan.
Two recommended plans are the following:
Both are easy to activate upon your arrival to the USA.
In Europe, and most other parts of the world, GSM (Global System for Mobiles) is the norm. When you subscribe to a mobile network, you are provided with a SIM card, and can easily switch networks by switching out the card.
The US doesn’t use the GSM standard as heavily however, and CDMA (Code Division Multiple Access) phones are common. CDMA phones don’t make use of SIM cards and won’t support non-US phones. In fact, phones are often restricted to a single network inside the USA.
There are 4 major networks in the USA, they are:
Only two of the major networks support GSM (AT&T and T-Mobile), but fortunately there are many MVNOs that resell access to these large networks which opens up some competition.