TerribleTom as in, how to make a cover for yourself?
if so, I would suggest getting a program such as Photoshop for making graphics. Photoshop does cost money, but there are plenty of alternatives out there for free, such as Gimp. However, I would definitely suggest downloading something rather than using an online program because downloaded ones are simply more... useful? things such as Canva are really, really limited. also, if you're willing to spend money, buying a drawing tablet is really helpful. (I would suggest a Huion because those are cheap & good quality)
from there on, you can google a bunch of tutorials such as how to use PSDs, how to blend, how to do photo manipulation, etc. etc. that'll be really useful for beginners. overall though, I would just say to get familiar with the program you're using. if you're using PS, the program is super complex and it'll probably take you a few weeks to get comfortable using it, especially if you haven't ever touched any other sort of drawing/graphics/art program like Photoshop.
haha hoped that helped! that's about the best advice I can give for how to start.