How do I disable a carriage return from submitting a form and how do I set focus?

I have a form with one field in it. If I enter a value in the field and hit tab, it just takes the focus off the field. I want the carriage return to do the same thing. Currently, the carriage return submits the form.

Also, when the user comes into the form, I want to set the focus on the field (move the cursor to it).

I know this should be easy, but I've searched for a solution and haven't found it. And I'm brain dead right now and under a deadline so I need help!

Thanks.