As long as you have a reasonable idea of how to drive a stick, you cannot permanently damage a new vehicle by being a newblet. Whether you buy a brand new car in a stick and drive it off the lot or just test drive one, you will be fine. If it is a performance car with lots of power, just use very little throttle. In all the vehicles, be very gentle on the clutch. As someone previously mentioned, the clutch is very "analog" and needs to be smoothly and carefully transitioned to produce proper starts and shifts.
You mentioned your dad has an old roadster or something: don't take it out on city streets or anything, but just ask him if you can drive it up and down the driveway or something. You can't break anything if you have a little instruction and are careful. You may not be very smooth, but a couple jerky shifts/starts won't break anything and will give you a wealth of experience to go test drive with.
Just get out and do it man, there's no other way. I too wanted to rent a car or something to learn, but that wasn't available. So with exactly two stick driving experiences under my belt, I picked up my new car and herky-jerkied off the lot. I was not smooth for a while, until a friend gave me some hands-on instruction on how to properly use the clutch. My hinderance wasn't even the difficulty of working the clutch, it was my own stupid preconceived notion that you just push the clutch in and out, when in fact you need to be smooth and at least at first, slow with it.
Good luck!