Everything depends on what you want to design, because that essentially dictates how you should approach that particular model. For example, an insect would take a very different approach from a dog, which would be very different from a flower.
There are countless topics on designing. Maybe look through those and see if you can find any useful information.
Personally, I choose the design method based on what I want to design (as I said earlier). For an insect or other arthropod, I use box-pleating, whereas for a mammal, I'll use 22.5deg structure. Experience has taught me what subbases and grid sizes to use for certain structures and proportions, so don't expect that to come very quickly.
An origamist's first designs are frequently modifications of existing designs, as Lang stated in ODS. So maybe try to add a graft to a frog base--this is only an example--and play with the resulting form for a while. 22.5deg bases are the easiest to experiment with, in my opinion, because most of the structures are relatively simple. If you fold a lot of 22.5deg models, you'll soon realize that many of them start with a few simple reference points. Start with those, maybe, and see what symmetries you can come up with.
If you need more help understanding how designing works, I'm sure many of us will be happy to oblige.