When looking at your mode of transport during your travel time, look at the pros and cons of each and make sure to find what works best for your budget.
Buses are often the most cost-effective way of traveling in the USA. Despite the cheap cost of buses, they can often be slow and are subject to delays and changes. Be prepared for instances such as this to occur.
There are many different ways to travel short distance by rail and many states will have their own railroad company for short distances. The national rail company is AMTRAK for longer trips. Trains are very efficient and clean, and provide a more comfortable, if a little more expensive ride than buses. They operate service to and from most of the major cities. Just like anywhere else, you should book in advance for cheaper fares.
Flying is the quickest form of transport while in America and internal flights are often very reasonable. Also look at other nearby airports as flight prices can sometimes be drastically different and the most obvious airport might also be the most expensive.
You are permitted to drive in the USA as long as you have a full and valid drivers license in your home country.
Most of the well-known rental companies will require you to have a major credit card (not a debit card), and be at least 25 years old. Some may accept younger drivers, but it may be more expensive.