You really can’t work without one! Who would hire a voice without hearing it first!
I’d say check out Bernard Shaw, he’s fantastic and an absolute pro. He also has a course about voice over at the actors centre.
It’s not as easy as most people think- I’ve been in v/o for four years and I’m still learning… you need to invest some time in listening to yourself recorded and sussing out what you’d be good for. You also need to look around at the competition to see what the standard is…
Hope that helps
Sally