In addition to the tutorial posted (which is great), I know some people find it helpful to keep in mind the information flow through the systems. Visualizing it.
Step 1: enter data (client side)
Step 2: javascript parse and confirmation (client side)
Step 3: Javascript "ok"'s it at the browser sends it off to the server
Step 4: ASP grabs the data using Request("") (server side)
Step 5: do any additional validation (such as checking the database for duplicate email addresses -- stuff you can't easily do w/ the JavaScript) (server side)
Step 6: process data (server side)
Step 7: display error/success message - either using Response.Write plain text, or you can also Response.Write some JavaScript out if you'd like it to say make a popup confirmation/error window (server side working w/ client side)
Good luck!