Buongiorno Giulio, benvenuto!
Good choice your AE-1, I also had it in the past. What I particularly like about this serie of cameras is that they had a winder that really matched the design of the camera body, without any ugly "handle" or useless "extra shutter release button". Since I sold my Canon gear I no longer purchased any winder, as I can't tolerate their awful design.
Coming lo lenses, you already got some good advice. In general, you want to avoid having nearly-duplicate lenses with too close focal lenghts, so what you have to do now is to fix a "multiplier" figure that will give you a rational progression of your lens set, for today and for the future.
If you don't think you will ever buy a large set of lenses, you may choose 2X. Then the 24mm and 100mm suggested by Alan Gales will nicefully complement the 50mm you already own. You may later add a 200mm.
If you really enjoy photography and you're quite wealthy, then 1.7X is another rational choice, and the 28mm f:2.8 and the 85mm f:1.8 suggested by Flavio are an excellent option. By the way this is the multiplier that I normally use, and these two lenses were part of my Canon set and I can say great things about both. Later, you may add a 21mm, a 135mm and a 200mm (not necessarily in this order, of course).
You don't need to buy them online, if you live in the Torino or Milano area I can suggest many used cameras shops where you will easily find what you are looking for.
