Are you already a commercial driver driving for someone else just considering owning your own truck?
Or are you considering a major career change from something else?
My advice if you are thinking of becoming a commercial driver for the first time would be to take it slow and drive for a trucking fleet for a year or so to learn if you want to continue. Used tractors are cheap to buy now in our bad and declining economy if you have cash or excellent credit and can buy one but once you buy it you'll have to give it away to get rid of it. Trucking fleets have failed and shut down and others have downsized according to what I have heard.
I've never driven big trucks but I spent two years and nine months, 400,000 miles, on the road as a commercial RV transporter. It was an adventure for me, I was retired with a pension and didn't have to work. Overall I enjoyed it. It was exciting every time I got a new load and a new destination assigned. But pulling trailers or driving a big truck on snow and ice covered highways in extremely cold weather is a high risk occupation. Drivers lose their lives every winter. Mistakes or mishaps usually come out of a driver's pocket. I have an old country music CD where the singer says, "the hours are long and the pay is low. " That's what most commercial hauling is. Some do very well, of course, but they have earned their way up the ladder. Not many do.
If commercial driving is going to be your livelihood, it is a tough life. Some drivers have worked themselves into driving for FedEx or Wal-Mart or a specialized fleet but most ordinary OTR drivers have to endure a lot to make a living. It is a life of sacrifice living in a truck on the road and in grimy truck stops, eating bad food, spending hours and days waiting to be unloaded or loaded, hanging around waiting for the next load.