To get a steady feel and sound it is important to anchor your hand. The best way of doing this is by placing the thumb on the top of the pick-up (see photo).
Most modem bass guitars have two pick-ups. One close to the fretboard and a second one nearer the bridge. In the early stages of playing it may be easier to place your thumb on the front pick-up as the string tension is not quite as "hard" as on the back pick-up. The tone will also sound fuller and heavier, and it will also be easier for your right hand fingers to play the strings.
You should also experiment by playing with your thumb resting on the back pickup, as the sound is clearer and has more attack and "bite" .