The hardest part is getting started and knocking on the first couple of doors. Once you get into the swing of things its actually kind of fun -- even if you are not an extrovert. Practice first on neighbors you already know if that helps you feel more confident knocking on strangers' doors.
I don't know what state you're in, but Ohio's website --
www.ohio4ronpaul.com -- has a good door-to-door canvassing guide as a downloadable word document. It's the "list of tips" link on the resources page.
Many houses won't have anyone home. I leave a slim jim stamped with my phone number on their doorknob (a hole-punch and a rubber band is the cheapest way to attach it.)
When someone answers the door, SMILE, MAKE EYE CONTACT, and say something like, "Hi! I'm your neighbor on X St, and I'm volunteering for Ron Paul's presidential campaign. Are you planning on voting in the primary?" If they say yes, ask if they know which candidate they are supporting. If they're undecided, ask what issues are important to them. Then talk about how Ron is the best candidate for THEIR ISSUE, and leave them a slim jim (I always point out the web address on the bottom and encourage them to check out his positions.) Be friendly and pleasant and thank them for their time, even if they're a Hillary supporter or a non-voter or call Ron a kook. Keep good notes so you can follow up later with likely undecided voters.
Again, the hardest part is getting started. Once you're out there, it feels REALLY GOOD to be doing something productive and helpful for the campaign. And when you find undecided voters who are interested in learning more about Ron Paul, you know you're making a HUGE DIFFERENCE for the campaign and the future of this country.