Use a false stitch when you are continuing with the same colour. To make a false tr/dc (UK/US), pull up a long loop, a little taller than a normal stitch. Place a finger on the loop on the hook and hold it firmly while moving the hook under, and wrapping the long loop around the hook.
Why is my granny square round?
The short answer is that stitches, particularly the double crochet, naturally slant to the right. When working in rows, that slant is offset by turning at the end of each row. When working in rounds, the instructions usually tell you not to turn.