This website contains training for all examination courses in KS4 and KS5.
Photoshop is used by professionals and is infinitely expandible via plug ins.
It is used by photographers and digital artists alike.
For your course it is important that you keep a digital diary of your inspirations and the techniques you used. I like the snipping tool in Windows for grabbing and saving only part of a screenshot which saves you having to crop and resize.
I will use PS as a short way of saying Photoshop.
It would be unusual to master a technique in one go unless it is really simple. Don't give up, keep trying!
Learn about layers, selections and file types first
for a solid foundation
There is usually muiltiple ways of achieving a task in PS. It would be useful to learn a few ways as each image may respond better to different technique
Don't follow tutorials in magazines and the internet mindlessly. Try to choose ones that explains why a particular too and setting has been used.