There are free tools available on the market which do a fair job of editing photos. Unless you are shooting professionally, I would strongly recommend to first use these.
GIMP stands for GNU Image Manipulation Program. It has most of the features offered by Photoshop; as this is not being developed by a multinational company you may find some rough edges, but the important thing is that it gets the job done.
GIMP can be downloaded from their website (https://www.gimp.org/ ). There is a fair amount of documentation available on GIMP website (https://www.gimp.org/docs/ ); I'd also recommend youtube tutorials by TheMalni (https://www.youtube.com/channel/UCO7PTo5j2jTkGleP95Q8skQ ).
Please do donate to GIMP team (https://www.gimp.org/donating/).
Photographers can choose to process the image in camera and store it in a compressed format called JPG, or they can capture the raw information captured by the sensor and store it in an uncompressed format called RAW for processing on a computer. Most photographers would recommend RAW as more knobs are available in RAW during post-processing.
Personally, I only click RAW if I have to deliver few high quality images; for fast paced shooting I generally click JPG.
Rawtherapee is a free raw editing tool. It can directly be linked to GIMP so that once you are done tweaking the image you can forward the edited image directly to GIMP for further editing. This setup culminates into an efficient work flow.
Download Rawtherapee here: http://rawtherapee.com/
Linking Rawtherapee and GIMP:
https://www.youtube.com/watch?v=-0qAurA4n40
Tony Northrup review:
https://www.youtube.com/watch?v=nWC-SAuYOzw
Affinity Photo is a great alternative to Photoshop. You'll be amazed at the things you can do with it and with how much ease. It costs around $49. I also found the general quality of final images better over the rawtherapee + gimp combination.
Buy here: https://affinity.serif.com/en-us/photo/
Tons of Tutorials: https://vimeo.com/macaffinity/videos
P.S. I am not paid to endorse them. :)