Now that you've determined your animal is a reptile (due to its hard, scaly, skin with scutes or bony plates), you're ready to look at other characteristics of its anatomy to further classify the creature.
This step is actually pretty simple. All you need to look for is legs. Either the animal has them or it doesn't, that's all you have to determine:
Has legs - Image A
Does not have legs - Image B
So what does this tell you? Well, you know the animal is a reptile already, and if the animal you're looking at has legs (as in Image A), it could be one of several types of reptile such as a lizard, turtle, crocodile, or tuatara.
On the other hand, if the animal you're looking at doesn't have legs (as in Image B), then it is either some type of snake or amphisbaen.