DIY aka do-it-yourself refers to the work you perform on your own, as opposed to taking it to a shop.
There are many repairs that you can do that will save money and also help you understand more about cars. The easiest way to determine whether you can do this work is to watch a video of someone else doing the work. It will help you understand what tools are needed, and whether you can accomplish the work using car repair videos as the reference.
The best place to get started if you are new to working on cars is the basic maintenance and repairs, which include:
Once you are familiar with the basics, you can decide whether to take on more advanced repairs:
There are manuals that outline the steps, however some of the manuals won't provide some of the most basic information that people need to get started doing DIY repairs for the first time. The solution is to find videos that are more recently published than the manuals and use both of them for reference.