Each amplifier lists these parameters - what speaker' impedance it supports, and what is power delivery over said impedance. Other factors are how many and what kind of inputs (phone, line/aux) are there, what is the frequency band supported (look for something at lest 40hz - 16kHz), what is the THD (this measures how precisely the amplifier duplicates the input), and probably - how it looks and sits with your other equipment.
You won't damage amplifier, or speakers, if it is rated for 4Ohms but your speakers are 6Ohms.