Depends on which platform. If you're using a laptop/PC to edit then I would try 'GNU IMAGEMANIPULATION PROGRAM' (commonly called The Gimp - https://www.gimp.org/ ). I use Linux and it works well. It's available for Mac and Windows as well - although I've not used it under those OSes.

I have heard good things about GIMP mentioned above, but so far I have always been happy with Paint.net which is free. Its only available on Windows though, and can be downloaded at https://www.getpaint.net/
Jase

I use Pic collage on my Android phone.
Simple but effective

Try this powerful app

https://play.google.com/store/apps/details?id=com.iudesk.android.photo.editor&hl=en_GB

I use Pixlr on my phone never had any problems, improves my photos no end!

Photogrid on android is a fab app.

I use Google Photos. It has an assistant that does some interesting stuff (e.g. videos, collages, this day X years ago...), it easily does some basic fixes to not that good pictures, and saves them all in the cloud with unlimited space. It does take some space on the phone, but if you clean cache and data from time to time, it is fine.

Photoshop always remains the best photo editing software on the market

Picmonkey is really good for beginners it's like a free and easier to use version of Photoshop.

Snapseed by Google is great, you can do some really cool things with the picture such as changing the perspective.
In terms of filters, stickers and other cool stuff I like ColorStory & A Design Kit. They’re both from the same company so work well together, and there’s lots of stuff on there, most of which is free!