If you don't already have an AMP login, you can request access here. Find the manufacturers whose catalogs you want to shop. They'll send you an invitation with a password to get access.
To shop on AMP once you have your login
1) Open your invitation email to find your username and password
2) Go to cms.amptab.com
3) Enter your username and password and click Log In
4) You will see your list of available catalogs, click Shop to begin writing an order