There are quite a few answers, but the first one is easy: Tell them. Or ask, if you want to be polite.
Start with your friends. And when I say friends here, I mean real life friends. The living kind.
Chances are you have friends on Twitter, Facebook or YouTube. If nothing else, I'm on those sites, and I could easily be your friend (as long as you don't like The Curious Case of Benjamin Button). So there--you already have a built-in audience. Tell them to follow you!
Or ask politely.
Then branch out. Invite co-workers, associates, peers, acquaintances... anyone you associate with face-to-face or otherwise. We all have a network, so start there. If you want more followers, ask more people to follow you.
Try this:
- Send an email to your friends, family, co-workers, or anyone else you think might want to follow you.
- Send a Facebook message with the same invitation
- Ask people, face to face, to follow you (harder to say no when you do it this way). Do it in small meetings or large gatherings. Like Christmas parties.
- Put your Twitter name on your business card or your email signature, and invite new contacts to follow you.