Well, I can't give you a real tutorial, but a suggestion:
1. Just take a screenshot from your screen by pressing [Print]
2. use eighter the program GIMP or PhotoFiltre
2.1 rightclick in an empty space in photofiltre -> insert as new picture
or insert it similar in GIMP
3. Just play around with one of these image editors, you will learn by doing

...and about Adobe Photoshop: Many people prefer it because of the many functions, but personally I have never tried it - too commercial and expensive (more than 1000€ for a bunch of bloatware)

There's enough great freeware.