Firstly, they are called fins, not flippers… Flippers a dolphin! lol
Fins for use in the pool should be simple slipper type fins. Do not purchase fins with straps and an open foot pocket for training in the pool… They come off easily and should be used later on in training under appropriate circumstances.
The foot pocket is generally rubber or silicone, and fits standard shoes sizes. Fit should be tight but comfortable without your toes poking out of the end.
The blade comes in many different forms but they all essentially do the same thing. For a first time purchase keep it simple just a simple blade without extras like splits, vents or springs.
Fins can cost anywhere between £50-£100. Theres no need to go crazy, especially if your young and still growing.