Yes, skins are a great way of getting started. I am still learning to mod too... currently I have made: A basic skin A colourable skin A colourable skin with non colourable sections License plates Now I’m going to learn to make a simple part with a simple jbeam, or work on a map... I suggest you should do something similar as you progress, taking it in steps is much easier
Personally I use Corel Paintshop Pro 2018 and convert skins to a .dds through a paint.net plugin. It’s just a matter of personal preference, depending what program you feel most comfortable with.
The paint.net plugin is free, Corel is not. However you can sometimes get Corel incredibly cheap in Humble Bundle bundles.
the paint.net isn't free tho, look https://www.microsoft.com/en-us/p/paintnet/9nbhcs1lx4r0?rtc=1&activetab=pivot:overviewtab