Hi, you need to use Live Paint Bucket. Here is a demo:
Draw a circle object
Next draw a line from the top to the bottom of the circle.
The line will be a separate path from the circle
The look will be a circle divided by a line into two parts, left and right
Now using the Direct Selection Tool select both paths
Then select the Live Paint Bucket Tool from the Tools panel
Choose your fill color and click inside one of the sides, this will convert the paths into a Live Paint object and that side will be filled with your chosen color.
Another way,
Draw your circle object
Draw your line above the circle
Select the line and from the Object Menu:
Object Menu,Paths/Divide Objects Below
This will divide the circle into 2 parts using the line as the cutting device.
You can then fill each new object as you like.
George
