In order to vote for Sam, you must first be on the electoral register.
Registering to vote is easy. Just go to www.gov.uk/register-to-vote. You need to be a British citizen or a Commonwealth citizen who has leave to remain in the UK or does not require such leave. You can even register if you are currently 17 years old, as long as you'll be 18 by election day itself (incidentally, the Liberal Democrats back votes for 16 year-olds).
If you have moved house, or recently changed your name (eg. after getting married), then it's vitally important that you update your details on the electoral register.
Similarly, if you think there's a danger you might be away on the day of the election, then make sure you register for a postal vote or a proxy vote so that you'll still have a chance to cast your ballot. You can do this at www.gov.uk/register-to-vote too.