Many new guitar players have a bit of a problem using a pick, and find it more comfortable just to use their thumb.
Playing with your thumb is OK, but you really need to get used to the pick, so you might as well start right at the beginning.
Hold the pick between you first finger and thumb so the point sticks out about a 1/4″. Any more than that and it becomes hard to control.
Angle the pick a slight bit so it slides easily over the strings when you make a downstroke.
Tuck your fingers under in a soft fist.
Keep your hand fairly close to the strings. And when you pick the strings try to keep your hand motion to a minimum.
Some guitar players rest their little finger on the body of the guitar to help stabilize their hand.
Get used to picking both down and up. Just rotate your wrist slightly to create a smooth picking action. When you pick a number of notes in a row you should try to alternate between downward and upward strokes. This is called “Alternate Picking”. Practice this motion. You will find it speeds up your picking as you progress as a guitar player.