It's a bit of a long process, learning how they work, finding one that suits you, then becoming proficient with it.
There are several free ones, but the only one I'd recommend would be GIMP, which can do pretty much anything that Photoshop can. Unfortunately it has a fairly steep learning curve. But if you are fairly good at technology and haven't tried others first, you might persist with it.
Until this year I would have recommended a paid app, Photoshop Elements, which is much easier to learn and will do as much as you need to do in restoring family photos. Unfortunately, with the newest versions your license for it is only good for 3 years. If you can get hold of an older version with a lifetime license it would be my top pick.
Perhaps start by downloading a couple of different free apps, see how they work and what they can do. If they don't suit, download trial versions of some paid ones to see what they can do, how they do things differently to each other and what ones suit you.
And, meanwhile, read lots of reviews about them.
Also, I'd recommend posting some of your photos on here for restorers to work on, asking what apps they use so you can compare results with your efforts on the same photos (and possibly with the same apps).
Hope this helps.
Peter
