To do a professional looking job is simple and easy once you know the tricks.
To fix your picture holes, buy a small bucket of all purpose joint compound (dry wall mud) using a small spatula wipe some over the hole and flatten out. 3 hours later come back with a wet sponge and wipe off the joint compound. The holes will disappear and be invisible. Don't even think of using sand paper this is a ridiculous waste of your time and makes more work and mess and is totally not needed in this situation.
To paint like a pro -
Clean the walls with TSP solution. Available at home depot or any paint store. Just get bucket of water put in the right amount of TSP and wipe off walls with a sponge. Then rewipe with old cotton t-shirts to remove any residue. This is easy and quick if it is taking too long you are doing it wrong. It should take 10-15 minutes to do a average room.
Prime walls if you are changing colors dramatically. If you use a primer do a good a job with it as you would with the final paint. If very dramatic color change have them tint your primer with your final color.
If you are not changing colors dramatically then just paint without primer. Use 2 coats no matter what how good you think one coat looks. The first coat covers the original color the second coat creates the final finished look and gives you your depth of color. Cut in first and roll on in "W" pattern. Choose your roller cover by the texture you walls have, the smoother the wall the thinner thickness of roller cover to use. If you are painting wood work another color consider adding some Floetrol to your paint available at home depot or any paint store, will take out brush strokes and leave a smooth finish.