Great advice from the folks above, which I've also benefited from.

Congrats on your decision--I just went through the same thing. So first of all, relax, you're in good company--we've all take a tumble.

I did it just the other day (red-faced), after heeding all the good advice here, I started out on my ride cutting across a little swath of grassy median in my own driveway--only to realize having rained the night before, the grass was more like bog--bike stopped moving forward, me and bike tumbled to the side before I could get my left foot out of the pedal!!

But once you're on your way, I gotta think you get the benefits of being clipped in right away! I love feeling both my legs working the cranks when I'm churning...