To add to what others have said...

Stand behind the bass and control its position with your body. Keep your hands free for playing rather than supporting the instrument. This will also help later if you want to use a bow, particularly a German one.

Put your left thumb on the back of the neck. There is always a temptation to grasp the neck between thumb and forefinger. This is one of those early lessons that will stick later.

Think in positions for your left hand but even when playing open strings, use your left hand to damp the strings between notes.

First position (first finger 1 tone above the nut) gives you scales of G and D without moving. A shift to half position (Simandl notation) gives you the low F for the key of C and top G# for A. A shift to Second Postion on the G string gives you top C.

Good Luck

Mitch