Assuming already that you have chosen your colors and the sheen level of the paint, you can now get started by repairing your walls. Start by taking a putty knife and walk around and knock off any high spots on the walls, also if there are holes in the walls, take the butt end of your putty knife and push directly on the hole, this will give the hole a slight indent around the edges. Now that everything is smoother and ready to fill, use 2 putty knifes, 1 to hold a small amount of filler on and the second to scrape the filler off the knife and to apply to the wall. Go around and fill all imperfections in the walls by applying a thin coat of filler to all defects, allow to dry and apply a second thin coat.
Applying 2 thin coats instead of 1 thick one will give you a nicer finish in the end , 1 thick coat will allow air bubbles to form inside and is also a lot more filler to sand off in the end. Remember, all filler shrinks , meaning if you fill a hole with 1 application chances are when it dries there will be a large concave dimple where the hole was, that is why its better to put 2 coats. Now that your filling of the walls are complete take a sponge sander, usually a medium to fine grit will suffice, and sand all spots using a circular motion starting at the outside edges and working your way to the middle, use your other hand to feel the spot making sure it is smooth with no ridges. Vacuum up all dust off ledges, baseboards, floor etc.
If there is baseboard and a door frames in the room, check around the edges and top for cracks where they meet up with the walls. Take a tube of caulking and cut a small piece off the tip to allow for just the smallest amount of caulking to come out. Use a quality caulking gun to apply the caulking, remember to just put the smallest bead of caulking where the crack is, have a damp rag on hand, take your finger and start from one end and push the caulking in with a swiping motion, it is best to dampen your finger first with the rag and then swipe the caulking off. There should be some left over on your finger, just wipe it onto your rag. Take your time with this step, its easy to get messy. If there is a large gap apply 2-3 coats of caulking allowing it to dry in between coats. Also, make sure you use a paintable caulking for obvious reasons.
Your room is now “prepped” and ready to go, just remove switch cover plates and cover the floor with drop sheets.