Vango, in order to become a good diagnostician, you need to have a fundamental understanding of how engines and vehicles work. This comes with time, so don't beat yourself up when you can't figure something out. Lots and lots of mistakes will be made along the way as you gain knowledge; that's how all of us got to where we are.
A common problem is a vehicle not turning over because of a dead battery. Most people then assume they have a bad battery, so they replace it. Maybe the battery was bad or maybe it wasn't, but they didn't take the time to diagnose to see if the battery was bad and if it was bad, why was it bad? Was the battery old? Did the ground have a poor connection? Was the grounding cable damaged? Was the alternator not performing well? Did the battery have a bad cell from the factor causing it to fail prematurely? Was an older but still functional battery failing because it can't provide the extra oomph needed because of the excessive demands of a failing starter? Etc.
Lots of stuff to consider. Therefore, pick a starting point. Start at the battery. Take it out, charge it up, and do a load test. Or you can use a newer battery tester, but those are expensive. Or better yet, many places will test your battery for free; that's my kind of diagnosing! Then let's say the battery tested poorly and is 5 years old. It's starting to get old, so you replace it and are done with the repair. (Or you could find out that the battery tests good, so you start diagnosing for why the battery failed to take a charge instead of becoming a parts-changer.)
The new battery is in, but some batteries last longer the 5 years, so you start wondering if another problem exists. Perhaps the alternator? Testing the alternator is quick and easy and many places will test this also for free. Yay! Then let's say the alternator tested weak, so you replace that, too. The alternator could have been weak and not placing enough of a charge on the battery, so the battery failed a little earlier than normally expected, or the battery could have simply worn out on its own. Or the battery was weak placing a consistent high demand on the alternator making it fail a little earlier than expected, or it could have failed on its own as mechanical stuff is wont to do. It's not unusual to find more than one problem when repairing your vehicle, so try not to have tunnel vision and try not to be short sighted and assume something is truly fixed after you've made one repair.
Ok. Now you're in a thinking mood. You notice that the vehicle isn't turning over quite as fast as it used to, so you wonder if the starter may be getting old. You've had the vehicle 10 years and haven't replaced it since you owned it, so you figure it's worth a look. You look and see that it is starting to show wear, so you replace that, too.
By this time you are probably frustrated all this stuff that has been repaired, but you take satisfaction that you replaced it now in the comfort of a friend's driveway on a nice day on your own time instead of on the side of the road in a rain storm. Diagnosing is being aware of how your particular vehicle works and starting at one end of a system and working through it until you find the problem, then not stopping there, but continuing your thought process to include possible other problems that may or may not be related to the one you found. Most people will let a problem continue until it utterly fails and then run around like a chicken with its head cut off trying to fix it; too much drama for me.
If you want to show off, learn how to use a multimeter. It's actually pretty easy to use, but you will amaze your friends and family on the stuff you can quickly and accurately learn. Get a good one, though. A cheap one, while helpful, is quite limiting.
