First off, eat and drink properly. Taking care of yourself physically is really important, especially to help you get over the jet lag (which I think is a big contributor to this weird initial period). Get nutrition in your body and stay hydrated, and get out and walk too! Keep your body working and your mind will follow.
Say hello to strangers. Just force yourself to talk to people, even if you really, really don’t want to. A simple exchange of words with a fellow backpacker will immediately make you feel better and will give you confidence in yourself. Plus it’s the only way you’ll make new friends.
Contact your friends and family back home. If you can skype or email them, do it as soon as! They’ll send you waves of support and encouragement over the internet and will leave you feeling motivated and grounded.
Last but not least, when your mojo finally decides to return to its rightful place, go out. A good night out with some new friends, whether it be heavy drinking or a relaxed meal, is so good for starting you off on your backpacking roll. Socialising is one of the best thing about backpacking and travelling anyway, so the sooner you start the better.