Not sure what exactly you want, need more details

The general format of a simple addition/subtraction formula is
=A1+B1
where A1 and B1 are the cells you want to add together.
or
=A1-B1
where you want to subtract the amount in B1 from the amount in A1.

As for balance, as in checkbook, you could have 3 columns involved:
A  used for deposits
B used for debits (checks, service charges, etc)
C used to show the balance
You don't worry about the first row where you'd probably only have a
starting balance in C2 (assumes row 1 used with titles for the columns)
then in the cell right below C2, obviously C3, you could put this formula:
=C2+A3-B3
that would take the previous balance, add any deposit put on row 3 to it and
subtract any debit put into row 3 from it.  You can fill that formula (search
Help for fill data) down the sheet to come up with running balance
calculations as for a checkbook.

If you need anything else, as noted earlier, we'll need more specific
information on how your sheet is laid out and what is where on it.