well, first of all you need to start off simple. simple designs and structures.
the more advanced and experienced you get in origami, the more complex origami you can design.
i first think of a model i want to create, then i look it up on google and look at every element the thing has and try and find a suitable folding sequence and at the same time draw rough diagram of it. then if you are sucessful, draw proper diagrams. it is natural to have truoble on a particular model and there will be much trial and error, but it's worth it.
also, check out arkive for more inspiration. designing models comes naturally to me, so it could be harder for the less experienced.