Since you are in the USA, you can use this simple method to purchase Bitcoins with cash:
1. Go to https://www.bitinstant.com/
2. Select one of the Cash Deposit options, and select Pay to Bitcoin address.
3. Enter your Bitcoin address in the "Destination account" field (if you don't have one you can go to http://nci2szjrwjqw2zbi.onion/
4. Enter how much you want to deposit (USD) and your email.
5. Follow the instructions. You want to pay by cash
, not check, otherwise you transfer will take 3 days rather than the usual 20-30 minutes.