Renting with the option to buy is often a misunderstood concept and is handled in a WIDE variety of ways. There can be a formal arrangement with legal contracts determining the ultimate sales price, stipulating that a portion of your rent will accumulate to be applied to your down payment when you buy, and your loss of those funds if you don't end up purchasing. This can be very risky for both the owner and the tenant/potential buyer. There can also be a less formal arrangement where you rent a property that the owner eventually would like to sell, and he simply agrees to ask you first before he sells it anyone else.
It has been my experience that people almost NEVER buy the property they've been renting. While they are living there as renters, they discover all kinds of reasons why they wouldn't want to buy it.
Your best bet is to simply find a place to rent for now, save on your own for the down payment and closing costs, and then when you're ready start looking for a place you would like to buy.
Best of Luck,
Keller Williams Realty