preparation to paint an internal concrete wall

[robag5]

having removed several layers of wallpaper from my new apartment, ive found the wall underneath is concrete. how should it be prepared for painting?

[bd3cc]

Dont think it will be concrete. It is probably an sand/cement render, used internally, which is then plaster skimmed.
How did you remove the wallpaper?
I suspect that you have removed the smikcoat in your enthusiasm to remove the paper.
This would not be difficult if it was not keyed properly, and blown.
I would have the walls reskimmed to provise a good surface for painting.