You can choose a winner using your own criteria, or you can use Hive to choose a random winner for you.
To pick a random winner:
View the entrants of your contest by clicking on Contests in the left sidebar on your dashboard > View Entry Report (the graph icon) on the contest you want to choose the winner for.
Here you can see data on all your contest entrants. If you click All Entrants in the Contest sidebar you'll see the Pick Random Winner button on the top right of the page:
Note: Clicking Pick Random Winner doesn't automatically notify the winner.
Once you've chosen a winner, you can click the paper plan icon to send them an email and let them know.
Here's more info on sending email: How do I send an email?