Seriously, you can do this now! If you are not sure what kind of dog you want or need to convince your parents that you are going to be sooooooo responsible, rent a dog from www.flexpetz.com.
FlexPetz allows clients to "rent" a dog, for a few hours or few days depending on their schedule, for a fixed membership fee. They can either rent the same dog every time, or try different dogs. The dogs are all potty trained (moms are you reading this!) and fully conversant with basic canine instructions. A membership also entitles members to rent pets from other flexpetz locations, when they are on vacation.
All the dogs at FlexPetz are rescued or in need of new homes. They are between 2-3 years old and routinely checked by veterinarians. When they are not being rented, they are kept in homes with one primary owner, just like regular dogs. The best part is if a renter falls in love with a dog, he can buy him and take him home forever. It's a pretty nifty way to try out if you really want a dog isn't it?
Source - Flexpetz.com