Painting is the highest-impact DIY upgrade for the money, but the result lives or dies on preparation. Most of a good paint job is what you do before the paint goes on.
Prep the room
Move or cover furniture, lay drop cloths, and remove outlet and switch covers. Fill holes and dents with filler, sand smooth, and wipe down walls so paint sticks. Tape off trim and edges if you don't have a steady hand.
Prime if needed
Prime bare patches, stains, and big colour changes so the finish coat covers evenly. A coat of primer also helps paint adhere on glossy or previously unpainted surfaces.
Cut in, then roll
Use a brush to 'cut in' a band of paint along ceilings, corners, and trim, then roll the large areas while the cut-in edge is still wet so they blend. Roll in overlapping passes and keep a wet edge to avoid lap marks.
Two coats
Plan on two coats for even colour and durability, letting the first dry per the label. Pull tape while the final coat is still slightly wet for the cleanest edge.