Its best to first remove the paper using steam, try not to use the scrapper too much - if the plaster is old you may chip or damage it by scraping. You should then sand down the plaster where it seems rough. Then fill in any holes before re-papering. In very old houses I do not recommend painting straight onto the wall. If you want the painted look, first paper the wall with blank paper, and then paint over. The paper will hid any problem areas.
Be very careful when using a steamer, we’ve had several incidences on the forum recently where this has caused more problems that it cured Lightly scoring the paper & using hot water is far less risky if it will shift it but if you decide to use a steamer, don’t leave it in one place for too long. If you overheat the plaster skim it will blow off the base coat, usually associated with light popping noises. You will then be left with even more problems which only stripping back & re-skimming are likely to cure.