Adobe is unfortunately the best. Whether Photoshop, illustrator, or Indesign. All have their purpose. Tons of youtube videos on the basics too. Their UI is generally the easiest to use and most straight forward Imo. If your computer can still run Creative Suite 6, it can be found quite easily 🏴‍☠️ and is very similar to the newer versions. The newer versions are more annoying to crack if you go that route, but it's still doable.

However, the best non Adobe software I've used is the Affinity suite. It's still paid, but they have generous trials without credit card, it goes on sale quite a bit, and I think you can buy it outright for a lot less than Adobe charged when they did that. It has alternatives for all three I mentioned above.

I'm sure the typical free and open source projects people mention work fine, but I never clicked with any of it. GIMP is terrible though.

In any case, just learn the ui and basics by trying to make stuff--or even try to copy someone else's design so you have a goal and can try to figure out how to do it. Like anything, just practice.

Even though the software differs, the concepts generally remain the same so you still do get transferable skills if you switch. You just have to relearn the UI and shortcuts etc..