First prime the walls with a good stain blocker primer like BINs. Then get a box of premixed joint compound and skim coat the walls (thin coat) use a 12" taping knife. Let this dry and lightly sand out any ridges with #120 paper. Do not try to get it all smooth the first time. Repeat the skim coats and sanding as necessary until the walls are smooth. Then prime the walls, any small defects can be mudded over and reprimed. Then paint. This will take a few days, do one wall, then the next, this way you will not mess up the corners, Or purchase a corner trowel, Just take your time, and make sure the compound is dry before you add another coat.