The first thing that you need to ask yourself when attempting to fix a computer yourself is: Do you have the know how?
Often, people will try to repair a piece of hardware yet they damage it even more. But, on the other hand, if you have a decent grasp of how the machinery works, then you can do it yourself. The internet is a goldmine of information and you can look up pretty much any problem to get information before you start.
Once you have identified what the problem is, then you can go about fixing it. Some things, like replacing a hard disk or a graphics card are pretty straightforward. You literally remove the old offending piece of hardware from the card slot, and replace it with the new one. Often, all it involves is slotting it into place and at most, screwing in a couple of screws. The same goes for power supply units on desktops. These are literally pieces of hardware that can just be unscrewed and pulled out. You can then just slot the new one in place, screw it in and connect it to the wiring internally, and off you go.
There are also plenty of step by step video tutorials online that you can watch to help you fix the problem. It is a good idea to watch them in their entirety before attempting to fix anything. As with everything else, if you are in doubt, then seek expert help. Most computer shops will offer servicing deals so you can put your mind at rest if anything were to go wrong. But if you opted against purchasing such a package, there are plenty of affordable and reputable computer technicians out there. Do be aware that if you open the casing on a machine, you will void the warranty. With laptops, it is always advisable to get repairs from the company itself.