Start point 4 sin wave

hi guys,

i have mc cruisin along a sin wave curve, code:

onClipEvent(load){
var position=_x=0,_y=0;
}
onClipEvent(enterFrame){
position +=1
_y=position;
_x=100Math.sin(_root.randposition);
}

Q: how can i set an orgin/start point?

thnx tons !!~~