I don't know of a web site that you can go to to download plans. Have you tried a Google search?
Let's try something else first. If you can read plans, you can draw plans. Get out the paper and old #2 pencil and think about this for a minute.
You know the size you want, (8 X 10), so you can lay out the box 8' X 10', depending what you plan to place in it, you should use 2 x 8s or 2 x 10s, Pressure Treated,(PT), for the base and floor joist. I would run my joist the shorter measurment, (7' 9"), 16" on center. I would use PT, plywood for the flooring. This is where all the moisture is going to be. After that you can go back to regular wood.
You need to decide where and how big you want the door to be and if you want any windows. (hint* if you want windows, go to your local home improvement store and see what they are stuck with, you'll save $$$)
Frame your walls, like you would any other wall and so on...
If you don't think that you are ready to design and build this yourself, you might want to go to a store like a Home Depot or Lowes and look at the plans they have. I think you will find plans for a shed in the Orange book at HD.
The most important thing you will need is an extra set of hands to hold everything in place while you nail it.