You need to reference your individual field labels. The easiest way to find out what your field labels are, is to turn on cforms tracking, make a form submission and look at the tracked fields (note: they're case sensitive). E.g.:

Dear {Name},

Also, if a visitor enters his/her full name,  how do I format the auto reply  email to  include  only the first  name as in  Dear  firstname.

Unless you use two input fields for first & last name, this is only possible via the my-functions.php filters and require

1. a hidden container (add hidden field) to temporarily store the firstname
2. have the filter dissect the user submitted name and extract a firstname
3. use the extraced name and assign it to the "hidden container" (via the data array passed on to the filter routine)
4. in your emai use the new hidden field label to reference the user name

…doabble, but requires some coding.