Yes, you must have experience before you really can design.
1. Choose the item that you wish to make.
2. Find an already existing pattern.
3. Follow said pattern once.
4. Look at the finished project and consider what modifications or alterations you'd like to make (different colors, decreases or increased, etc.)
5. Then experiment! But remember to RECORD your progress or any problems that come up, you'll want to remember these when you're finalizing your new pattern.
For example, I'm trying to make globes for earth day. I'm modifying this pattern http://i-like-lemons.blogspot.com/20...nit-balls.html
to accomodate intarsia for a miniature Earth.