For full-length to cropped at the waist, a 50mm would be OK.
For head and shoulders shots, anything from 70-200 would be OK
A good-quality zoom would probably suit you best unless you want to go the expensive route and buy high-quality prime lenses.
If you're hyper-critical about quality (and I mean hyper-) then Primes are probably the way to go, but it'll cost more.
A good-quality zoom lens allows a range of focal-lengths to be used, so something between 50-200- I'm sure someone more familiar with Canon's range of lenses will be along shortly to give you some exact examples...
I'd buy two lenses: a 'standard' zoom and a telephoto-zoom; something like a 24 or 28-70mm and a 70-200...
Apparently Canon do quite a selection around those focal lengths, so finding ones to suit your budget shouldn't be a problem - and if new lenses are out of your price-bracket, consider a good used item...