So knowing absolutely nothing about the bike's history or condition beside this post, I can list some probable bare minimum to get it running again

1. Chain. The chain looks like it's dead, new 9 speed (educated guess) chain will should be less than 20 euros or dollars
2. The tires look old but not terrible in condition. If the tubes are not holding air, swap them off. ~5 western currency per tube (times ten if you're scandi)
3. Cables. I see the rear derailleur cable is detached, and by the overall condition of the bike I'd guess it'd be the time to replace them to make sure shifting and breaks are working properly. The cables will likely cost you ~20-30 freedom or eurocommunism coins

There may be some bearings and whatnot rusty or loose, can't tell from picture, but that's what I'd start with. If you'll bring the bike to a mechanic, they'll probably bill 30-50 scamcoins per hour for the work, depending on where you live. With just what's listed above, should be 1 hour work unless there's a lot more stuff to fix

If you can rattle the bike around a little bit and tell what feels strange and what feels normal, or what you've felt you don't like, please tell and can maybe try to be more helpful :)

Edit: also much love for wanting to bring new life to an old bike <3