Step 1: Ok, we'll start with the head!
1. Draw a yellow circle with a brighter gradient in the middle.
2. Using the Fill transform tool, move your gradient to the right upper corner, like in the second picture.
Step 2: Now for the mouth!
1. Draw a simple triangle shaped mouth using the line tool.
2. Now, using the selection tool, bend your lines to your desired shape.
3. Add extra lines for the tongue and shading.
4. Color it in!
5. Remove the lines..
6. ..And put it on your original head! There, you have a good looking mouth for your smiley.