How Do I: Sum Of Selected Columns & Linking Columns

Currently I'm building tables and forms. My first table (called Clients) lists the details of fictional clients. My second table is for invoices.

In my invoices table, I wish to link the column for client reference (note: stored in the Clients table) to the column that precedes it. This column will list the clients’ names and is selected from a drop down list that is linked to the Clients table.

What I want to do (if its possible) is to have the respective client ref. automatically show up in the next cell once I've selected the client to whom the invoice relates?

Am I making sense? Is that possible? If so, how do I do it?

Secondly, how do I do a sum of selected columns for my “totals” column? Basically, I want to add the figures found in several cells that precede it?