If you are facing problem to create categories in Blogger, then the mater is so simple. Because the categories for your posts is known as Labels in Blogger. So if you want to specify a post of your blog to the desired category then simply type the Name of that category in Labels field just below your post compose area.

You have to assign/create a category only once, next time whenever you are going to post regarding to same category then you do not need to re-type again. Just click on Show All and you can see the all categories created by you. Then you can select your desire category by simply clicking.

If your post meets the requirements of more than one category, then you can assign all the related categories to that post by selecting or if new category, then by typing the name separate by comma (,).