Simply dragging to colours from the swatch panel to the gradient panel will make a gradient. Adjust the colour placing to where you want and choose between radial or linear.

Once you have a gradient you like, drag the gradient to the swatch panel, now the gradient can be used again at a later time.

Here are two squares with two types of gradient. The first is radial, the second is linear.

By selecting the gradient tool we can drag the gradient over the selected shape to get the desired angle wanted, like below.

Now we can go to the next tutorial on using the pencil tool.