Without knowing your detailed layout I can only be generic but here's the idea:
Say you have 3 fields [Text1], [Text2], [Text3] and you want to keep [txtTotal] updated. For EACH field that contributes to the total, you'll need to do this --
1. Right-click the text box in Design view, select Properties. Click on the Event tab.
2. In the blank After Update line, double-click to show Event Procedure. Now click the Builder (...) button to go to the code sheet. Enter this:
[txtTotal] = Nz([Text1]) + Nz([Text2]) + Nz([Text3])
Save the form and try it out. The Nz wrapper converts null or empty values to zero so Access can add them up -- otherwise it tends to choke.
Denis