The Amazon EC2 interface for selecting an AMI is totally unmanageable. Everything else in the GUI is reasonable, though, so I prefer it to doing it all via the command line tools. This is nuts:

First, decide what Ubuntu version you want to run and find the EC2 AMI for it. 

Start here: http://releases.ubuntu.com/

I chose the latest and greatest, Maverick Meerkat, here: http://uec-images.ubuntu.com/releases/10.10/release/

Now when you start your image in AWS, filter by the AMI ID & the selection becomes much easier:

This guide is great: https://help.ubuntu.com/community/EC2StartersGuide . If you started your instance in the AWS GUI, it's easy to see the command line steps that correspond to what you did in the GUI.

Note that with the Canonical Ubuntu images, you'll need to ssh as the "ubuntu" user: