I am making a 3D letter and I want to fill in some parts to give it a 3D look. So far I have used black lines to make it but I want to fill in-between the lines.
Can someone show me what to do?
I'm going to assume here that you're not using the pen or shape tools. In this case you'll want to use the live paint tool. Select your design and press the K key on the keyboard to activate the Live Paint Bucket Tool. Then choose a color and start filling.
In the future you may want to use the pen tool. This will give you more control.