The first and most important thing is to work out where the damp is coming from. Until you have shut off the source, you’re likely to continue having problems.
But this may be easier said than done and will often require some detective work. The damp may be coming from a leaking gutter and finding a route through the walls to come out in your bedroom, or it could be a problem that has to do with driving rain getting into cracks, or possibly a porous external surface.
Another very likely possibility is that the damp is not coming in at all – it may well be condensation forming on the inside of the walls due to poor insulation and a lack of good ventilation.
Once the source of the damp has been shut off, the building fabric will need to dry out completely – a dehumidifier will speed things up and can be rented from plant hire shops. When this is done, the remedial plaster/paintwork can be done with confidence.