Most origami starts with a base. It could be a very simple base, like the classic bird base from which cranes are folded. Or it could be a very complicated base, like the one that Hoang Tien Quyet created for his horse and is published in the book, 50 Hours of Origami by the Vietnam Origami Group. 

There is a thorough explanation of origami bases in Robert Lang's treatise on origami design, Origami Design Secrets.

So the first part of origami design is figuring out which base is most suitable for what you want to fold. Again, Lang provides many tips for choosing a base depending on the number of arms, legs, wings, etc that you might want for your model. It is also really educational, in my opinion, to learn from other designers, especially in the beginning. Acquire as many origami books and publications as you can and Keep Folding. Identify the base the designer is starting with and think about how each fold transforms that base into the finished model.