Rectangles

Select the Rectangle tool button, then click and drag out to draw rectangles

You can change the fill and stroke (border). You can also change a rectangle in having rounded corners by increasing the amount for Roundness.

You can adjust shapes using the Scale/Skew/Distort tools.

Hold down the Scale tool button to see the following options:

A rectangle is a special shape, you'll see it's called "Rectangle" in the Layers. If you try to adjust the points using the Subselect tool, it'll give you an error message. You have to ungroup it first.

Once it's ungrouped, it will say "Path" in the Layers.

One way to get a triangle, you can start with a Rectangle, then just select and delete one of the corner points.