You probably don't need a primer. If the current wall is smooth, clean and covered with the same type of paint (both are oil-based, for example), you can go straight to get the paint. You need to fill holes, putty and sand*, maybe even cut a new piece of drywall on the existing wall. Always print walls before painting if the surface is porous.
The surface is porous when it absorbs water, moisture, oil, odors or stains. For example, new drywall is a very porous material. Both the paper that covers it and the mud for drywall are compromised by water or moisture when they are not first sealed with primer. This material will literally absorb paint directly into it if you don't print it first.
Apply a primer before painting the walls. A primer is essential if you want to go from dark walls to light or white walls. Many brands offer paint and primer in one. If you're painting a wall that hasn't been painted for a long time, it's not a bad idea to prime it, although you could also get away with cleaning it thoroughly.
Fresh drywall absorbs paint like a sponge, and turbid joints take on paint differently than bare drywall between joints. If the wall you're painting is damaged by water or is otherwise stained, you must prime before painting. To help achieve a uniform appearance with the final coat, when painting new drywall or on drywall patches, it's always a good idea to use a primer first and, in addition, using a quality drywall primer is usually much cheaper per gallon than using several coats of latex paint for interiors of quality. You'll achieve a smoother, seamless look by mixing the wet paint you applied with the wet paint you're applying.
If you're thinking of painting over those old wooden panels in your study or painting a set of shelves, you'll want to prime first. Learning to paint a room can involve mastering the steps for painting walls and ceilings, as well as careful preparation and cleaning. And once you paint over the primer, you can clearly see areas where the paint has spread too thinly. You can buy a medium purpose paint sprayer or a professional paint sprayer or, as an alternative, search for rental options online.
If you're applying flat paint to a wall that was previously painted with a high or semi-gloss finish, print first. Now that you know the proper steps to paint a room, get the supplies you need, such as paint roller covers, paint trays, and more. That is why paint manufacturers have developed a new, faster drying technology for oil- and latex-based products that dry quickly and, at the same time, help the finishing coat of paint to adhere properly. Painting a light color over a dark color requires additional layers of paint to prevent the old color from being seen through the new shade.