stretching theories can clash but i think the accepted wisdom now is don't stretch cold, tight muscles (before you ride) just warm, loose ones (after the ride). Like mattp said, ride slow to start with and after 10 or 15 mins at steady state, when you feel the heat coming to your legs, stop and gently stretch out your calves and hamstrings at least.
A really good stretch session after a ride will make a big difference to your healthly long cycling life. We do coaching for an awful lot of people now, from recreational riders to elite racers, and the one thing we see across the board is that those who have a good stretching routine (not necessarily a long one) suffer less injuries, have better posture on and off the bike, and usually pick up new techniques more quickly too.
Best way to learn some good stretches is either find a yoga or pilates class, get a book or scout the internet for ideas.
Finally, if you have never stretched then you may be harbouring some old muscle damage, knots or tight spots that can really be helped by sports massage. I confess to having been a cynic about this until two years ago when Tom in Penrith started fixing me with his lazer accurate massage. Makes such a difference, every now and then.