American Here..
You can get a T-Mobile or AT&T SIM at any carrier store. You are not required to sign a contract or have a SSN (Social Security Number) if you are only going for month-to-month service and you are not financing a phone.
T-Mobile is great, I use them (since they have unlimited roaming in ZA), but they don't have the best coverage (Think Cell C). If you are planning on visiting national parks or going off the beaten path, go with AT&T. They are a good value with several Prepaid plans that would meet your needs.
AT&T is a bigger, more traditional, telecom company with coverage mostly everywhere, but we hate them since they're big and they don't have to care (Think Vodacom).
Verizon and Sprint are CDMA/LTE carriers, they don't use GSM/HSPA for their 2G/3G so many phones won't work on their network. If you have an unlocked iPhone 6 (or newer) you will be able to activate a device on their network, but it might require jumping through some hoops. This is because Apple phones are dual-mode and can use a CDMA or GSM based network, with LTE on top. Verizon is the biggest carrier in the country, but they can be hard to work with. Sprint is a train-wreck and not worth the time, so don't bother. -- I can discuss this for hours if you want.
If you are looking for the best value, go with Cricket (
http://www.cricketwireless.com), they're an MVNO (Mobile Virtual Network Operator) owned by AT&T, so they use the AT&T network. They offer great value, and have a truly unlimited plan for $60 for a month (unlimited voice, data, and sms). The SIM will work in a phone or a hotspot (even though they say it won't) and is truly unlimited but limited to 8mbit download on LTE, 4mbit on HSPA (3G). You can purchase an activation kit at any "big box" store, Walmart, Target, etc and refill cards at the same place. It is truly unlimited, I've pulled 60+GB in a month over it and nobody has said anything.
We don't require anything like RICA, so you can visit the store, buy the SIM and refill with cash, and then activate online from your hotel (or any free wifi, it's everywhere).
The industry in the US does things very differently than ZA. Some things are better, some things are infuriating.
-Jamie