Where to start..?
First of all, I've found it best to stick with 8-ohm speakers. That way, two of 'em can either be 4 or 16 ohms (in parallel or series), one of which is bound to work fine with whatever outputs your amp has. And four of 'em will be 8 again. It's also a lot easier to mix & match if they're all the same impedance, and it'll be easier to sell if you don't like it as well.
As far as which speakers to start with... well, my friend, it sounds like an easy proposition, but it's a real rabbit-hole once you start chasing it. I've managed to generally cover all the bases with speakers, but I've got a complete ***load of speakers & cabs. If you want some classic "essentials," here are some good suggestions:
"British" (Marshall/Vox) style:Celestion G12 Blue AlNiCo (or equivalent)
Celestion G12M "Greenback" style (or equivalent)
Celestion G12H-30 (or equivalent)
Celestion Vintage 30 (or equivalent)
Celestion G12M-65 (or equivalent)
"American" (Fender) style:A couple o' Jensen P10Q (AlNiCo) or C10Q (ceramic) (or equivalents)
Jensen P12Q or C12Q (or equivalent)
Jensen P12N or C12N (or equivalent)
Others:Tone Tubby H1E, AlNiCo or ceramic
JBL D120 (or equivalent 'cuz they're rare)
EVM-12L (or equivalent)
...and there's probably far more suggestions like those above. Keep in mind, also, that I say "or equivalent" because there's so many clones of speakers (and clones of clones!) that it can be confusing & bewildering out there. But the good news is, if you have any questions, you can always ask everyone around here!