The elliptical? (the exercise machine)?

Well, first all I can say is that it's extremely hard for me to use, I use it for about a minute at a time, its so hard on my body, well really just my legs. and i see people get on those machines for 30 minutes plus and go at it no problem but its like as soon as i get on it, it hurts, Very bad. like i can bearly move. and everyone says you can work with your whole body with it, like someone told me one day they do there upper body, and the next they do there lower. but i don't understand how you do that. i get on and the only place i feel the burn is in my legs. so whats up with this? and i do have allot of muscle in my legs, so i don't see how this is so painfull to me, my sister got on and she said it just gets her heart going, it doesn't bother much with her muscles until after wards....so im wondering if im doing it wrong, or if maybe there's something i can do to alter the machine to make it less.. pain full. oh and the whole upper-body/lower body work out.. how do you switch? thanks, deff ten points to whome ever helps me the most! =)