In Spore, users will create their own species from scratch, starting from the microscopic stage and evolving them through subsequent phases (up to five, according to Electronic Arts).
Spore Creature Creator is just a trial version of the creature creator in the full Spore game, so it won't include all of the resources you'll eventually have available.
Creating a new being is as easy as adding different body parts from a lateral menu. One thing to keep in mind is that you have a limited amount of credit to spend. Mouths, ears, horns, eyes, arms, legs, scales, claws... everything has a price. It's also possible to give your creature some color. You can use predefined styles if you don't want things to get too complicated.
Once you've finished constructing your specimen, you can give it a name and test out its movement to see if it will succeed on the ground, and how it responds with various emotions. The creatures that you create will remain stored in the Spore Pedia. Don't miss a peek at the examples that the game has provided you.