Kimonodonya has a great posting explaining how to wear a kimono. I've reproduced the section explaining how to tie an obi here for future reference:
With your left hand, hold the obi about 40cm from the end (tesaki) and fold in half lengthwise, open side downward.
Wrap the obi around your hipbone, not your waist, twice. Hold the other end (tare) at about the same length as the tesaki and slide it inside the obi from the bottom until it sticks out from the top.
Tie the tesaki and the tare together so that the tare folds over the tesaki.
Fold the tesaki diagonally.
Hang the tare over the folded part.
Bring the tare under the tesaki and tie tightly.