Did I hear that you're looking for an emulator? I recommend Hatari [0] if you want to run games and demos. You'll likely need a TOS ROM image [1]; Hatari comes with EmuTOS, which doesn't work with a lot of games that make assumptions about the memory layout of internal OS structures and the like.
If you want to run well written GEM applications then I recommend ARAnyM [2], which is a lot faster, due to not attempting to do a cycle-accurate emulation of all the original hardware.
And if you have any issues with the above, there are still lots of people hanging out in Atari-Forum that might be able to help [3].
[0] https://hatari.tuxfamily.org/
[1] http://www.avtandil.narod.ru/tose.html
[2] https://aranym.github.io/
[3] https://www.atari-forum.com/