Depends what connectors you have on your ethernet card on Amiga. Older cards may only have AUI and/or BNC connectors, but newer have modern RJ-45 standard connectors (which your PC also has presumably). If it's old without RJ-45 (phone jack style) then it's better get it converted to RJ-45 with AUI transceiver for example.
Cable should be standard RJ-45 network cable then. If you want to connect it directly computer to computer, then you need cross over version of it. If you're using switch or router, then get normal straight through cable. Switch/router is better if you want to connect both computers to internet with broadband.