How can I draw a circle (or circular shape)? Every time I try to draw a round shape or curved line, it spirals around and does random lighting-bolt-like patterns.

Chances are, your lines are set to spiral towards the center. On the computer version, open the settings and click on "Spiral towards center" to turn it off; on iOS, tap on the tilde (~) on the top right of the screen and select one of the straight lines (–) that appears in the menu to disable the spiraling. Make sure you're not keeping your mouse or fingers stationary for too long, too, as that will cause your lines to start "spinning" in place.