Painting a room is one of the easiest things you can do to enhance your surroundings. Its mostly fun, fairly inexpensive, and can be a very satisfying accomplishment if done well.

It's a Long Job
Painting a medium room takes about 12 hours of work. If you rush in, trying to get a room painted in one night, you are going to end up exhausted with white doorknobs and hardened trails of paint dripping down from forgotten nails. You might be able to get it painted in one night with three friends, but if you can't recruit three friends, plan on it taking a few days at least.

Choosing a Color
The paint department of any hardware store will have dozens of free color cards, known as "paint chips", to bring home and tape up in the den.

Buy some 12-ounce test paints if you aren't sure how a particular color is going to look on the walls back at your house. These cost $3.50, which is a lot for 12 ounces, but worth it if you end up not liking the color.

Glossiness
In addition to color, you'll need to choose a glossyness of the paint, either non-glossy "flat" and "eggshell" to "semi-gloss" or "glossy". I recommend painting most rooms with eggshell walls, flat ceilings, semi-gloss trim.

Bathrooms and kitchens usually get semi-gloss walls and trim.

Equipment
You need a roller, a roller pan and a brush.
If you are starting from scratch, buy a painting kit, which will provide the minimum tools required. Buy more rollers if you are recruiting friends to help, but you might just want to share the first one, because the more you use, the more you have to clean.

Don't clean the walls
When I learned to paint, I was taught to clean the walls with a TSP mixture. But then the main ingredient was banned, so no one sold TSP anymore, so I started using TSP substitute. Finally, I decided that washing (and rinsing!) the walls first was a huge wet pain in the ass, so I stopped doing it. I haven't noticed any difference at all in the final result, so when it comes to washing the walls before you paint, my recommendation is: Don't Bother.

Move Furniture
Take down blinds, curtains, outlet plates, clocks, frames, mirrors, posters
Don't leave any residue from adhesive squares or double-stick tape. You can't hide that with paint.

Its possible to paint around one or two big pieces of furniture, but it will really slow you down, especially if it is going to impede your constant trips to the roller pan.

If something appropriate is available, keep one waist-high piece of furniture in the room to use as a work table. Cover it with plastic film or a tarp. You need something in there, or you'll end up putting all of your tools on the ground, which is not a good way to work.

Protect the floor and windows
Tape off stuff that you don't want to get paint on. Use masking tape to tape down one edge of a roll of paper. If you are planning on painting the whole ceiling, spread out a drop cloth or plastic film to prevent damage from spills.

I usually use an 8" wide strip of paper along the walls, and plastic film to cover the rest of the floor. I also use paper under the door, because I always get a little sloppy when I'm painting big trim elements.

More preparations
Set up lights - If you are painting at night, you'll need extra lights to get a good look at the paint as you put it on, to see details and where the old paint color might be showing through.

Fix Holes
Paint won't fill in holes. Fill anything bigger than a sewing needle hole. Don't paint over nails.

Use a tube of caulking to fill long, thin gaps, such as the gap between walls and moulding.