Running stitch is one of the easiest hand sewing stitches, and the one you will you probably use the most! Just two simple steps repeated to form lines and lines of stitching!
Stitch up through where the yellow circle is
And then back through where the blue circle is
Repeat these two steps, making sure your stitches are even length and even distances apart.
You can then use this stitch to fill in shapes like illustrated below, either with spaced out, irregular stitches or closer, dense stitches to fill the spaces differently.