On your main time line write:
var lifeline = 100
make sure that your dynamic text box has the variable name 'lifeline' minus quotes. It can be found near the alignment buttons, thats where you write it's name. Then for subtracting health, i'll keep it simple. make a button and put this on the button:
on (release) {
if (_root.lifeline>0) {
_root.lifeline -= 50;
}
}
Everytime you click it your health will lose 50 and not go below 0. Hope that helps.