Nothing is ever simple in After Effects. There are many ways to do most everything, so to make a half circle two come to mind. So you as the artist must choose which is the best. First would to draw a full circle then mask half to leave your half circle. Second would be use the pen tool to draw a belzier curve in the shape of a half circle. Good Luck.
You can draw circle and then select mask tool and draw rectangle over the circle to cut it as squidkings said.
You can also draw circle, and then in the same shape draw rectangle, take merge path and substract it.
You can draw it in Illustrator and then paste it into AE.
You can draw circle and then apply linear wipe effect to it.
You can do all kinds of things with Merge Paths in AE. It takes about 2 seconds. Draw Your shapes (keyboard shortcut q and press multiple times to cycle through the options) using the shape tools with no layers selected, arrange your shapes, Select either contents or the shape layer and then click the little add button and select Merge Paths. Spin down the Merge Paths options and select an option. The only disadvantage of doing this is that the Merged Paths will all have the same fill and stroke colors.
If you want to have multiple colors for your shapes you will have to use multiple layers and track mattes. This also just takes a few seconds.
So I will need two different colors. Can you walk me through the steps a little bit more? I am very new to After Effects and while I am used to layers in Photoshop, they seem to behave differently in AE depending on layering and selections. Thank you for your help.