You need your CDL license from the DMV. It's a special license that allows you to drive a Semi. It can be a hard profession. Trucks are expensive and even more expensive to repair and maintain. You always have to figure in that the price of diesel fuel fluctuates.
It is difficult, but potentially rewarding. First of all, you obviously need to get a commercial driver's license. Next, you have to buy a truck (they are extremely expensive). You will also need a lot of operating cash. Fuel is very costly, and semis break very regularly; repairs to them are quite expensive.
It depends. There are companies that will hold your hand and help you with all the necessary paperwork and day-to-day operations. If you go at it alone then it can get difficult. You have to get your own loads, pay for maintenance, etc.