Illustrator combining shapes

I'm new to illustrator, please help me with this. Imagine you are having a cylinder and then you use the pen or curve tool to make an arc in the corner of the cylinder, which connects to the path. now i want to colour that part alone,( the part that wil be like a triangle at the corner, i hope you get what im saying) but i can only fill the pen tool's arc not as triangle. In what way should i proceed which will let me to fill them as a triangle in corner not as arc.