Keep your wrist turned so that your thumb faces the ceiling. Twisting your wrist when throwing will result in a curve or "hook" as the ball approaches the pins.
First and best advice is if you have ever pitched slow pitch softball or throw a ball underhanded that is the basic bowling form. Put your thumb in first and the 2 middle fingers in next. When you release the ball your palms are facing out.
take the ball in your dominant hand and walk up towards the line swing underhand palm up towards the pins try to take a slight angle at them not straight into them.let go right as your arm gets out just past your knee.