I recommend using clad quarters first and then clad half dollars. Clad coinage is the coins we use today. Their metal value is much less than their face value (except nickels). Most are made of nickel cladded copper or copper cladded zinc. (NOTE: Clad coinage will turn fingers green due to copper reacting with skin. Also, some people have a reaction to nickel). These are inexpensive and the best to learn on. When you can make a consistent clad quarter ring, move up to a clad half dollar. once you have that figured out, move up to a clad dollar like an Eisenhower dollar.
Once you have these coin rings under your belt, then move to the silver quarter, silver half dollar, and finally the silver dollar in that order. Once you have silver figured out, then move into gold and then finally platinum. The risks increase as you move up in the value and rarity of these metals but so do the rewards.