There are two basic types of leather Chrome tanned normally used for clothing and furniture and often very flexible and Vegetable tanned (veg tan) leather which is used for a far wider range of items, including Belts, Wallets, Camera cases, Horse tack and so on, and also used for Tooling where you basically engrave leather using hand tools or stamps
I would suggest you start with a single shoulder of Veg Tan with a thickness on about 1-1.2 mm which you can use for wallets and other small items to learn on
Veg tan leather can be very thin say 0.6 mm pig skin for lining bags etc to very thick for traditional shoe soles. It can also be very soft and bend easily or stiff like thick cardboard so the best way to buy if you have a dealer near you is to look and feel before buying
Some dealers also offer economy leather which is far cheaper and often used for training, this often will have marks or bites that make it unsuitable for first class presentation but great to learn on
Hope this helps