Never used credit before, want to make my first credit purchase

Pay back on time and never miss a payment. If it is a loan then generally the early payments are mostly interest. The interest to principal ratio of each payment changes over time. At the end of the loan period your payments are mostly principal so if you have extra cash and can pay towards the principal early on in the loan you can reduce the amount of interest you pay over the course of the loan.

Credit card interest rates are generally high so carrying a balance gets expensive.