I Am 15 Week Pregnant & Recently Diagnosed As Low

I am 15 week pregnant & recently diagnosed as low lying placenta. My doc advised to take rest and to lift the foot end of the cot when lying.I need to know detaily about what precaution should be taken in such case. Can i sit for long time or can i walk. Or I need to be in bed for whole day.any problem will happen if i sleep on supine position.