Painting central heating radiator
The radiator is probably steel. To remove the paint, you can sandpaper it. 

Optimally, you should dismount it, so you're able to get to the back of it as well. If you dismount it, you can also take it to a workshop and have it sandblasted. That's the quick way to strip the paint.

As for painting: If you get down to bare steel, apply a primer that is suitable for steel, and a coat of a paint that is compatible with your primer to give it color. This can be spray paint, which will give it a uniform surface, or you can brush it on - which will probably give a thicker, less uniform coat. 

For the pipes, the same approach should work. Be careful with the amount of stress you impart on them, as this may lead to leaks further down the road.