Trial will give you access to 2 gb memory and many services for 30 days. If you wish to continue after 30 days, you will need to give a credit card number and switch to a Pay As You Go
Pay As You Go will remain "free" as long as you use only one container, stay at 512 mb of memory and do not go above any predefined limits of any of the additional services. You can click on the link that says "Estimate your costs with our calulator" to verify.
You can also select Pay As You Go instead of Trial, but the instructions below assume you have selected Trial
Click "Start your free trial"
Enter the information required (email, etc.) and click "Create Account"
You should get an email shortly from IBM that you will need to verify