name/model of receiver -> google -> specs
name/model of speakers -> google -> specs
compare the two.
you don't need to match them, if your receiver says 4-8 ohm then anything between 4 - 8 is fine.
I've never tried to drive anything more than 2.1 system but I'd imagine it'd probably be best to match impedance of all your speakers so the center channel isn't either way louder or quieter than the others, but wait for someone to chime in because it might not even be an issue if it's controlled separately.