Stores such as GK Elite sell underwear made for leotards and competitions. They also sells things that are shaped like leotards so they fit under your leotard and cover that part of your body. These don't show underwear lines and are made mostly in nude colors so that even if it does show, it won't be so noticable. You may also be able to find some regular unnderwear that doesn't show many lines.
No, usually tights are worn in place of underwear and then a skirt or tutu over the leotard to cover up the top of the line from the tights. Wearing underwear underneath a leotard is a big no-no, especially in performances. Pantylines look horrible.
Two answers in one....lucky you. You don't have to wear underwear if the leotard is "panty lined, or you wear one of those body suits under neath. You know the ones I mean? The ones that slim you in the process and don't show any lines / seams
this is a no no in many dance and cheer schools and competitions! this is because panties like to come out of the bottom of the leotard and if you tuck them in it makes a bump. if you are wearing tights it isnt needed and if not wear a thong to prevent panty lines.