I recommend watching a few different videos about fretting-hand technique, since different instructors are likely to give you different advice -- especially went it comes to thumb position. This is the one that was a real game-changer for me:
This is a good video. It doesn’t hurt to watch it several times as you’re learning. A lot of guitar players who pick up a bass naturally come over the top of the fretboard with the thumb. This can inhibit the free movement of the other fingers. The bass is a different instrument with longer frets and wider string spacing. What works on a guitar can quickly lead to pain after prolonged bass playing.