Next, find a tape deck. Best case, a nice one like this with RCA outputs that's in good condition. An old Walkman will do if you're not picky.
If you're buying one new and you're thinking of converting many cassettes, consider one of the many tape decks being made today that include a USB output specifically for the purpose of digitizing cassettes. As an example, I reviewed the Alesis TapeLink USB a few years back. The tape deck shown above (Ion Audio's Tape2PC) is also a popular option.