For me, marking wild amphibian specimens while running "research" just for fun at the age of 14 is like being an Arnold fan and injecting steroids without training at the gym. Go outside and do herping, read books, make photographs of the animals you find, understand them, but believing that as a scientist you will use data you collected as a 14year old kid (counting? measuring?) is, well, funny. At your age and younger i used to make projects of terrariums in my notebook. I can barely look at them now and i'm only 20.
You can still find interesting animals outside in the woods but taking a good picture is likely all the "documentation" that makes sense. Take a pic, note when and where, maybe the size, that's all. You need no statistics and tables. If you don't have a good camera, spare money to buy one!
And if you are going to spend all your teenage years in the woods catching and observing amphibians, you need no "data collecting training". They will teach you this and many other boring things at the university, with your practice from earlier years you will rock at the beginning of field lessons.
