Repairing the body of vehicles in Car Mechanic Simulator 2015 is the easiest job in the game. The only catch can be if you have to repaint it. For that you need to have purchased the Paint Shop for your garage. This is also a great opportunity to stock up on spare parts for the future.
Finding the damaged body parts can actually be a little difficult with a simple visual inspection. Some vehicles are in great shape, and the problem is that they are slightly less than perfect. The up side here is that when it's a body job, all of the parts will be listed. So just check the work order under body, and you'll see the complete list of parts to be replaced.
A tip here would be to replace the parts. Don't bother with repairing them. This will add spare body parts to your inventory. You can then repair these later, preferably with your skill at 90%. Then you will have a nice little horde of spare body parts for buying cars at auction and rebuilding them.
While not part of the game in the beginning, they have added a Welder to the Garage. You'll find it along the wall next to a locker. Check the images below.