You should get into either USAFA, ROTC, or OTS and earn a pilot slot.  Once you have a pilot slot, you need to get to a UPT (undergrad. pilot training) base that graduates pilots to bomber training.  For that, you will need to be good at college (to get the right base) and good at UPT (to graduate to bomber training).  In bomber training, you also need to be very good.  There are very few B-2 slots available because there are so few of them.  B-2 pilots are usually more senior pilots.  You may need to fly a different aircraft first, then move to the B-2.  It is not a very common job, so you will need to work hard and perform well.