Given the general nature of your question, I can only offer generalized advice on the topic.
The general rule of thumb for expertise in any subject is 10k hours of deliberate practice. This applies to both photography and photo editing, so count on 20k for both. It is subject to the 80/20 rule, so you can become competent in about 2k hours. You'll never stop learning though; I've been using photoshop since version 2.0, and I still haven't mastered every aspect of it.
Concerning software, if you're looking form something low cost, Gimp is a good program, though it is limited compared to its pay counterparts (primarily Lightroom and Photoshop). I lean toward the Adobe stuff because it's so widely used that there is far more learning material available.
Since you've mentioned people who sell their work, beware of the Dunning-Kruger effect. You'll learn a lot very quickly and it can be easy to think that because you know way more than you did, you're now an expert. In reality, you're a novice who has not yet learned just how much there is to know. It takes a while to get to the point where you know enough to have a realistic assessment of your own level of skill. Diving in to selling your work too early is a good way to get both a bad reputation and derail your desire to learn. Basically, once you've gone from thinking you're amazing to thinking you're terrible again, you're finally making real progress.
The best ways to learn the basics are to take some online classes. Several sites offer complete entry and mid-level courses on photographic editing that are very inexpensive.
The general advice I give the photographers I teach is to spend your time mastering photography itself before you start worrying about post production. Not only will that knowledge make you better at the artistic side of photo editing, it will reduce the amount of editing you have to do. The temptation will be to use post processing to fix mistakes you make in taking the pictures. It takes far longer to fix a photo in post than it does to take the photo well.
As Deke McClelland said: "Photoshop is not to make bad photos good, it's to make good photos great."