There is no real short answer. There are many things that will determine this. What size rod, single or double handed? What size fly? How are you fishing it, top bottom, or mid level? How fast, clear, and deep is the water? Lastly, what are you fishing for? All these will come into play. If you want to keep it simple to start, just divide your fly size by 3 and this is the correct size tippet for the fly you are using. Next thing to answer, do the fish spook easy? If yes, use fluorocarbon; if no, use mono.