need an some experienced help with EXCEL formulas

[Schugg]

im trying to set up a spread sheet the takes all my checks, bills, payments, balances and what not, and calculates everything as i enter it month by month. i got all of that done and it works great, but then i was like ****, just cause thats what i pay thats not actually whats going towards the balance due to interest. so i wanted my balance figures to be right so im trying to figure out how you make the formula that takes the monthly payment and the interest rate and does the math to get the amount of the payment that is going towards interest, then when it calculates that i can get it to calculate how much of the payment is going towards balance. i have 5 rows for each bill/debt row one is the payment, 2 is the addition payment (payment ontop of minimum payment that interest doesnt apply too), row 3 should be how much of payment is applied to interest, row 4 should be how much of payment is applied to balance, and row 5 is the end balance of the payment minus the interest i was charged. i had something working with calculating it, but i dont know if i just dont know how to calc. interest or if i did the formula wrong, cause it wasnt coming out like i thought it should. i dont really know how to show you what my sheet looks like on here without send it, so im assuming if your experienced enough to figure out what im trying to say. or if you really want to put an effort into helping i can send you my sheet so you can see how my sheets layed out and what my formulas are.