Buying a semi truck in some ways is like buying any car -- you have to be careful about your purchase and consider all of your needs. However, buying a semi is also much more of a business decision and can be much more complicated than buying a personal automobile.
Make a business plan. Big trucks are big business, or at least they can be. You need to plan your purchase around the idea that this is your first and biggest business decision. You need to have a business plan that will help you and the bank know how you will be able to afford your rig. At the very least, you should have some jobs and/or contracts lined up to prove a large enough income to pay for your truck.
Chart out other costs such as gas, insurance, repairs and taxes. Take your business plan and a copy of your credit report and shop around for a good loan. The companies that sell the big trucks can offer financing, but they often charge a higher interest rate.
FInd a dealer. Once you have secured funding and have a good business plan, you need to find a semi truck that will work for you. Pay special attention to hauling capacity to make sure the rig can do the types of jobs you plan on taking. Look in many different places. Go to different dealerships and find what they have to offer. Consider going to out-of-state dealers. Out-of-state dealers usually can't directly compete with each other but they can sell to anyone. You may find a better deal farther from home.
Be willing to look for used semi trucks. New or used, you need to get a specification sheet that lists everything the truck has. You may not be able to find or afford a semi that has everything you want, so you will have to determine what is most important to you and what will best help you fulfill your business plan.
Pay special attention to the age and mileage on a used truck. Semi trucks are worked hard, and you need to consider the repair costs that might be higher with a used truck. Take a used truck for a good test drive and verify the fuel efficiency. If you can, you may also want to have the truck looked over and diagnosed by an independent mechanic. You will also want to make sure that your rig is comfortable. If you're getting a rig with a sleeping or housing compartment you will likely be spending a lot of time in the truck, and being comfortable is important to doing well and driving well.