BackgroundMost professional photographers use Lightroom and Photoshop from Adobe. There are other very good proprietary programs out there too.You cannot buy Adobe image editing software anymore. They are available now only by a monthly subscription fee. It it $10 USA dollars a month? I'm not sure. For professionals who maintain thousands of dollars of equipment, phones and pay monthly studio rentals this is not a significant cost. Adobe products are as good as it gets.
For many of us an additional ten dollars a month is a burden. There are some very good and constantly improving free open source image editing products available.
Open SourceIf you shoot images in RAW format (I highly recommend you do) You will need, at minimum, a RAW image editor and Gimp. Gimp is the General Image Manipulation Program. Gimp was the first free editor to become available. It is still very good and in fact, for open source development, indispensable. But you need an RAW editor too (gimp can handle many different image formats but it cannot deal with and RAW formats).
Jpeg OnlyIf you are content to shoot Jpeg only--and don't want to struggle with RAW image format files--then Gimp is all you need. You can lighten or darken images a bit, adjust contrast some and you will want to learn how to manipulate the "Levels" menu. You won't be able to change Jpeg images much but for some people that's enough.
Raw EditorsIf you are interested in skiing the black diamond image editing slopes you will need to have a good RAW image editor as a first step. Adobe Lightroom and Photoshop provide that and do it well apparently.
I'm a free open source guy so I work with Darktable and Gimp.
From the RAW editor you will save a new file in tif format and then open that interim *.tif file with Gimp, where you will do a few final touch-up steps. Most of the work gets done with the RAW editor. The two most powerful and most popular free RAW image editors are RawTherapee and Darktable. RawTherapee is probably the best choice for beginners. It is very good software and it is easier to get started with.
Darktable is harder to learn but substantially more powerful. Darktable has a Lighttable mode that competes with Adobe's Lightroom. Lighttable is used to export images but it also imports sorts and organizes your images, complete with a one to five scoring mechanism that makes it easier to find your favorite images. I only use Lighttable for exporting images but many find the database-like storage mechanism valuable.
RawTherapeeRawTherapee can be installed on all three systems: Linux Mac and Windows. A "RawTherapee" search at https://youtube.com will provide many instructional links, including installation and use. => RawTherapee Website <=
DarktableDarktable can be installed on all three systems: Linux Mac and Windows. A "Darktable" search at https://youtube.com will provide many instructional links, including installation and use. => Darktable Website <=
GimpGimp can be installed on all three systems: Linux Mac and Windows. A "Gimp" search at https://youtube.com will provide many instructional links, including installation and use. => Gimp Website <=
Gimp is a free alternative to Photoshop. Beginning users can do just as well with Gimp as with Photoshop. The entry-level image manipulation instructions given here will apply to Gimp only. There are many very good Photoshop tutorials out there on the web.