A year ago I was after doing the same thing and had no idea where to start. Well then I got to thinking about the usual character builds. So I made myself a character sheet and labeled it for a Monster. Just to see where it may lead me.
My advice would be to Make a CS, put down what type of monster your thinking similar too to start. Then try and develop a basic appearance, powers, strengths, weakness. Why they are a monster, what made them, or even how they came to be?
Once you have a start on this for an idea. You could try drawing on paper if you like art. If not ask a friend what they maybe see or even look up different pre-made monsters from movies or even other artist. After that the building a final product is easy.
The monster I made is basically a mix between an image from the creatures in Homes for Particular Children that most can not see. Then I added creatures body parts to get a structure. By the end it was a lion, Komodo Dragon, Gorilla, Goat, and Praying Matus build. Let's just say it was the first and only A I had gotten all computer art semester.