Principal vs Interest Mortgages

[Parsa Izadpanah]

Hi, I was reading Brandon Turner's Rental Property Investing book, and he was talking about principal and interest mortgages. It made perfect sense to me until the part where the amount of interest and principal changes. I just want to know if the first month let's say the monthly payment is $500 and I put $400 of in interest (My profit) and only $100 of it for the principal (the loan payment) don't I still have to pay $500 a month? How do I exactly profit from it? 

Thank you,

Parsa