It can be confusing to know the proper terms to use, especially when you are talking about your computer. Diagnosing means that you are trying to get into the root of the problem. For example, there is something wrong with your computer, but you do not know what is causing the issue.
You would like to diagnose your computer so that you will know exactly what is wrong with it. Troubleshooting refers to the things that will be done in order to fix the problem that has been diagnosed.
There are some people who are particularly good at diagnosing and troubleshooting. If you know that this is something that you have never done before, it is best that you will look for a professional who can do both tasks for you.