Explaining how to sex a rabbit online is a very hard thing to do(at least for me....-hehe-) but I found a website that explains it pretty well! There are a ton of other websites and books you can look at if you still are lost. Well, here it is:
http://www.rabbit.org/chapters/san-diego/health/sex.html
How to Sex a Bunny
Determining the sex of a young rabbit can be tricky, but very important if you want to pair up bunnies too young to be neutered without getting an un-wanted batch of babies out of the deal.
While seated, place the rabbit's feet on your chest. Hold the crown of the rabbit's head in your hand and bend over from your waist and place the rabbit's ears between your knees, with the rabbit reclining securely in the wedge between your legs. With your index and middle finger, press gently on either side of the vent (genital mound) area.
If the rabbit is a male, a tubular protrusion will be seen. Even easier, if the male bunny is about 10 weeks old or older, testicles will be noticed on either side of the genital mound. If the bunny is a female, the protrusion will look more like a slit. When the rabbit is old enough (4 to 5 months) be sure to have it spayed/neutered.
Here is another site's info:
http://islandgems.net/sexing.html
Determining sex:
The sexing of rabbits, at a very young age, is comparatively simple, but like many aspects of rabbit keeping, it is best learned by practical guidence from a knowledgeable person. Apart from the sex organs, there is generally no differance between a buck and a doe except a buck generally has a broader head and is the smaller of the two. The easiest way to sex a rabbit is to hold the rabbit's head with your palm, and gently wrap your fingers around the rest of his/her body.(you should be sitting down if you have a larger rabbit so you have a place to rest your arm and the rabbits body) With your other hand, use your thumb and index finger(or if its more comfortable use the index and the middle finger) and gently press either side of the rabbits sex organ. The organ of the buck should appear more rounded while the female's organ should appear to be a slit or V-shape.
With increasing age these differances should become more and more apparent. With a mature buck the scrotal sac should usually be apparent, but it should be noted that the buck can pull his testicles into the body cavity, and the apparent abstence is therefore not a certain guide.
Well, I dont know if these helped much, but you should be able to understand a little better then you did before you read it! If you need anymore help I'll be sure to try to answer in my own words, giving you what I know from experience! Well, good luck and PM me if you need to!!!