apart from the usual poor ventilation there are only two types of damp rising and falling so check your guttering pointing and windows for falling ingress and check that you have a good damp proof course for rising check it is above the outside ground by at least a few inches and that your cavities are free from rubble etc this way no damp can penetrate - once you are confident there is no further problem it depends on the wall but if its a good sound plastered wall strip back the paper to the plaster and give it a GOOD dose of bleach and degreaser mixed together with hot water ( something like washing up liquid or anything that says it strips grease) if you can leave it to work for about 1 hour go back and wash it off later ( remember to work in good ventilation and wear gloves - good for bathrooms and very black mouldy window frames - good luck have fun