Calculating Sums

How do i calculate the sum of several fields and have that sum generate in another field?

Also, how do I set fields to only "show" and accept data if other fields are selected. For instance: in my database I have created a communication form for contacting either clients, third party clients, or company contact. How can I set it up so that there is a drop down for those three sets of data which then generate the drop down list from the set of data selected. I hope this makes sense.
ayishagisel