I was nervous on my first shoot too. I forgot how to work the camera, and wasn't paying attention to the model. I was also using a new camera that I wasn't familiar with.
Below is a shot from my first shoot, and feel free to critique it if you like. Notice how the breasts are tack sharp but her face isn't? I was using a new camera with autofocus and didn't know about the autofocus lock feature. When I focused on her eyes and recomposed, the camera refocused. Had I remembered to keep my finger half depressed on the shutter release I'd be fine. We forget things on first shoots....so stick with equipment you know and are comfortable with.
The term speedlight might be a Nikon term, but it is the flash. The 420 I'm sure will be fine, but you'll have to talk to some Canon guys about it to be sure. Just about anything will work, but if you don't have a flash meter, you'll want one that is compatible with the TTL features of your camera. You should be able to pick up a used one for a decent price.