You'll flat the back wheel more of than then your front, at least in my experience.

Buy a bundle of tubes, even new bikes get flat tires, so if you replace your bike, you'll still need the tubes at some point.

Learn to change your own tube, do what the others have said about checking the tire for debris or something stuck in it. However, I'm assuming your first tube went flat due to it being 4 years old, and the second one sounds like was pinched between the tire and the rim when it was inserted. An easy way to avoid that is to pump up the tire to about 10-15psi and push the tire around the beed, all the way around the tire to ensure the tube is in the tire and not pinched between the two.