It's really good to keep your blog organised and categories give you an excellent way of doing that.

It's OK if you make up categories as you go along. If something is similar to something that you wrote before, if not make a new category etc.

You can also have child categories within a parent category, so there are many different methods for you to use.

I wouldn't worry too much about categories. Obviously use them and make it easier for your visitors to navigate your site, but don't let them confuse you, you're in control!