Expression in forms

I am new to Access and I have a Table with a field called total.I have
created a form to enter or modify data .I put an expression in the form in
the Total field .The expression adds the results of several fields in the
form and I want to add the total to the Table total field.The expression adds
the field in the form but I can't get the field in the Table to update.I am
not sure how do do this.not even sure what expression to write and where to
put it .I gues what I am looking for is a clear step by step proceedure on
how to do that. I realy need help