How To Tie An Obi
Obi are usually 26.8 cm (approx. 10.5 inches) wide and 3.6 meters (approx. 12 feet) long. You might need to make it longer or shorter depending on your size; it needs to go around you twice with extra length to tie the bow.
To wear the obi, you fold it in half lengthwise. This is for kimono obi, mind you--yukata obi are half the width of kimono obi and are not folded in half prior to wearing. For a kimono obi you also need a number of accessories depending on the style of knot you're doing--for an otaiko musubi knot (very popular knot; you see it on married women often) you'll need a bustle sash (obi-age), bustle pad (obi-makura), various strings (koshi-himo) to hold the obi together while you're tying it, a support panel that goes under the obi to keep it smooth and unwrinkled (obi-ita), and an obi cord (obi-jime). You may also need some narrow towels for padding under the kimono; your figure is ideally supposed to be a perfect cylinder, and padding is often used to fill in the waist area and any other hollows so as to achieve a smooth line. Yukata are much simpler--all you need for yukata is the yukata itself, the half-width obi, and some geta. ^_^
If you don't want to have to struggle with the obi every time you put it on, you could make a pre-made bow which attaches to the obi separately. These are popular with girls and women who don't know how to tie obi. Basically it's two parts--the part that wraps around has little strings on the end which you tie and tuck into the back of the obi, and then the pre-made bow has a metal loop which hooks into the back of the obi, and strings which you bring around to the front and tie, then tuck into the obi.
If you want to tie your obi the traditional way, there are literally hundreds of different obi bows and knots, and tying obi is an art in itself. I only know how to do a couple of styles, but here are some useful sites:
Instructions for folding an otaiko musubi knot for a kimono obi. FYI: after you wrap the obi twice around yourself you should have a long end and a short end. The long end is the 'tare', the short is the 'tesaki'.
Here's an animated gif instructing how to make a bow for an obi. They're using a kimono obi in the picture (kimono obi are double width), but the process is similar with yukata.
This diagram showing how to tie an obi for a hakama (swordsmen's pants) is actually almost exactly how to tie a yukata obi in a bow, except that you wrap the yukata obi around yourself a couple times instead of once.
It may be difficult to figure out from the images, so here's a description too. This is describing how to tie an obi for a yukata (unlined summer kimono):
Leave about 3 feet of obi hanging over your shoulder and then wrap the rest around your waist. Wrap until you have about 3 or 4 feet of leftover (not the part over your shoulder). Basically you should have slightly less obi over your shoulder than leftover in front. Take the part in front and fold it in half lengthwise. Take the piece off your shoulder and do a simple overhand tie with this piece and the other piece; pull the top end up and the bottom end down to secure. Take the shorter piece and put it over your shoulder for now. Now take the longer piece, spread it to its full width, and fold the excess length inwards. This part will be the 'butterfly wings' of the bow. Now take the piece over your shoulder, make sure it's folded in half lengthwise, and wrap it over the middle of the butterfly wings, bringing the end of it up underneath the knot. Pull it tight to keep the bow secure and then tuck the end into one of the folds of the obi itself.
Once you're sure everything's secure, carefully slide the bow around to the back (make sure to slide clockwise so your collar isn't pulled out of place), and your'e done! But be warned, it'll probably take a bit of practice before you know exactly how much obi to leave for the bow and such.
Make sure that when you're putting on your yukata that the left side overlaps the right. Lots of Westerners make this mistake; right-over-left means you're a ghost. o.o;
When I have time I'll draw little pictures to illustrate this better. ^_^