Welcome to Bytes, Stephen. The first lesson everybody here at Bytes has to learn is that good questions usually get good answers quickly and fuzzy, unclear questions may not be answered at all. Your question is pretty fuzzy.
Should we assume you know the equations for computing interest? You don't say if you are computing simple interest or compound interest. You say "compute interest" and "and then apply at end of quarter" as if they will be two different steps.
We don't know if you'll be doing this on a form, one account at a time or in a query across the whole number of accounts you have.
The best thing I can tell you, if you want to stop being not very good at VBA, is to spend some time with the sample database that came with your copy of Access. Try to set up a form to calculate interest on one account at a time; then write a query to calculate for everybody at once. The sample database is very good for helping a novice learn a little and we, here at Bytes, can help you get past the things that don't seem clear.
When you come back with a clear, concise question be sure to tell us what version of Access you're using.
Jim