Step 1:

Start a new document of size 250x250.
We can create our shapes using the following tools

Step 2:

Let's draw some shape using the rectangle marquee tool

Use the Pain Bucket tool and fill some colour into the box

Step 3:

Let's draw some shape using the eleptical marquee tool

Use the Pain Bucket tool and fill some colour into the circle

Step 4:

Let's use both rectangle and eleptical marquee tool to create other shapes

This is a bit tricky

draw a rectangle first and hold alt and draw circle over it using the eleptical tool and fill it with the paint tool
I got this

if u see clearly u can see the circle that i drew. I did it so that u get a clear picture.
so that's the trick holding alt to remove the part u don't want and u can hold shift to add the circle to the rectangle.

let me show u what i got by holding shift when drawing a new circle over the rectangle.

that's all now it's on keep playing with various shapes.

here are some more examples