You can register the domain for as long as you want.
A yearly domain registration is fine. Although I've heard that having a longer domain registration will improve search engine ranking but that's just speculation, no one except google knows for sure if it does increase ranking.
But if you plan to keep the domain anyway and have the money then it can't hurt to register it for 5 years or for the maximum length of 10 years.
A 10 year domain registration cost at most $100.00