step one:find the person who put your bike outdoors and, as you're "quite built", threaten to hurt them badly if they don't help you unfreeze your bike components.
step two: Use the penetrating lube (don't giggle) liberally, and let it sit around on the frame for a while. Like, overnight. Have your dad/old roomie/whoever put the bike outside help you as you use various tools and body parts to try to dislodge parts, caveperson style. (Brute strength, primitive strategies.) Do this for, like, 3 hours.
step 3: if, at anytime, the frustration overwhelms you, just thank your lucky stars that you got the seatpost out.
step 4: if this goes on any longer than, say, 3 hours of caveman shhh, consider getting a new frame.
Take a wire wheel to the exposed part of the steer tube to attack the rust scale so the spacers stand a chance of sliding when dislodged. Spray more WD-40 around that mess and let sit. Then pound on it with a rubber mallet fist and try to drive out the steerer. Then use a big ball-peen or a small sledge when that doesn't work.
BB cups should be less of an issue, since that should be aluminum against aluminum. Just use a good BB cup socket and a long 1/2"-drive breaker bar.