Well, I guess PhotoShop is a pretty obvious one.
But since you're inexperienced, you may be better off with something more simple if you're not aiming high.
Photoshop and Gimp are very similar but the main difference is that Gimp is completely free and open source. It's a bit less user-friendly however and doesnt have as many features as Photoshop.
Illustrator is also made by adobe and complements photoshop very well when making designs from scratch