Welcome to Subsim,
riccidp !
Usually you do not download a game here, you buy one somewhere, and here is the place for enhancemants, patches and "corrections" that turn your bought game in a completely new and more realistic experience.
On advice which simulator to get, this is up to you. Contrary to some 20 years ago, there is now quite a bunch to choose from.
There are very few WW1 sims, more WW2 sims, and very few newer (like covering the time of the cold war) sims, that are up-to-date.
Fast Attack is a real good one, but very old and has some problems (crashes now and then), and the only real new one to be played on a good PC would be "Dangerous Waters" (am i right?)
If only someone could make something like "Fast attack" based on modern PC technology, and graphics.
There are also some to be played on a Pad/tablet PC or on a smartphone, but i lack the overview, in this respect.
Have fun, and greetings,
Catfish