Using shape builder in Illustrator allows you to start creating iconography and begin your illustration adventures.
First, create the shapes on which you want to apply the Shape Builder tool.
Next, using the Selection tool, select the paths that you need to merge to create the shape.
Then, Select the Shape Builder tool from the Tools panel or press Shift+M.
The Shape Builder is really three tools in one. To join shapes or paths, just click and drag over your selection. To separate shapes, simply click on each individual shape. To remove (or erase) parts, hold down alt/option.