Posting a charge at the first of the month

I have about 3 weeks experience with excell and have set up a simple spreadsheet to show customers on a trash collection route. The customers rates are not all the same so the amount they pay is in one column. The amount they have due is in another column and the amount paid in another. I need to post the monthly rate to the amount due column on the first of the month adding it to whatever balance they may have and be able to subtract any payments made from the account too. Any ideas how to set thid up? I am just beginning to understand the way functions and arrays work from reading the book I bought from this site. It is a little hard to learn it overnight though and I sure would appreciate and jumpstart I can get

Thanks
Grant

jan feb mar
customer b/f cost paid bal b/f cost paid bal b/f cost paid bal
tom 0 10 0 10 10 10 10 10 10 10 20 0


the balance is b/f plus cost minus paid
the second and subsequent b/f's are set equal to previous balance values

I appreciate the help but as I mentioned earlier, I am a total newbie to Excel. Is there some way you could draw me a picture like they do for preschool kids. I see what you posted and know what you mean but have no idea how to put it into the spreadsheet to make it work. Just think of me as someone who does not speak your language at all. (Excel language) I know, get someone else to do the spreadsheet but then how will I learn?

Thanks again