I have two speakers with 8 ohms 91dz and 100w rms (250w max). How much watt and ohms will my amplifier need? I lack basic understanding of electronics and therefore need advise on the minimal requirements an amplifier needs to not destroy my speakers.
8 ohms is nothing, you don't need an amplifier for that. The power supplied with a mobile phone is enough to drive your headphones. You'll blow out your eardrums with an amp. (250 ohm headphones and I'd recommend an amp, but you could still get away without using one at that level)
If you still insist on getting an amplifier for your headphones, you can get really any amplifier. Just make sure you don't go and max out the gain (I wouldn't turn that up too much anyway because it can end up amplifying a signal enough to hear some of that annoying white noise). Again, The decibels required to actually wreck your headphones will wreck your ears before you destroy your headphones.