Painting your walls is one of the easiest and least expensive ways to make a huge impact on your space.
Unfortunately, that impact can be positive ("wow, the room looks so much bigger") or negative ("why is there a big purple stain on the carpet in your bedroom?"). However, if you take your time and pay attention to what you're doing, painting is a fairly easy skill to pick up (it just takes a little practice... and sometimes some trial and error).
I'm hoping that this instructable will help anyone who's new to painting. Whether you want to set yourself up with some good basic gear to tackle your fixer upper, or you just want to get your dorm room or apartment painted. In a pinch, I'd say you can paint an average sized bedroom (without major drywall/plaster issues for a little less than $50US (including paint, primer, sandpaper brushes, and rollers) and the price can shoot up from there.
Step 1: Figure out what you're doing.
First of all, you need to figure out what kind of paint you have on the walls. If you use a little denatured alcohol on the paint, it will tend to have a noticeable effect on latex paints and it won't really do anything to oil-based paints. You often also tell by feel, latex paint will tend to have a slightly rubbery give to it, oil will be smooth. However, figuring this out by feel usually requires a fair bit of practice so if you have any doubts, go with the chemistry approach.
Why is this so important?
Well, you want to know what kind of paint you have so you can either put the same thing on top of it, or so you use a primer that will let you switch to a different type of paint. This is most often done in going from oil to latex paints. Latex paints are preferred for general wall painting because they can be cleaned up with water and are therefore much easier to work with, but they really don't like sticking to oil paints (at least not without some help from specialty primers).
Once you know what you have on your walls, you can go to the store and talk to the folks there to find out what they recommend for your situation. One thing I would add though is that you shouldn't feel like you need to buy the most expensive paint out there. I usually go for "one coat" store brands. You might need to use more than one coat, but they do tend to go on better than your average paints. More expensive paints do have somewhat better qualities for painting with them and in some situations do last better once painted, but IMO they aren't that much better. If you're painting something that you may want to repaint soon-ish (e.g. dorm room) you might want to seriously consider saving some money there. That's a personal and financial preference though. However, if you can, you should try to buy yourself some good quality brushes, rollers, etc... If you take good care of them they can last you many many years and they really do make a difference in painting. However, a perfectly reasonable alternative is to buy something very cheap and plan on throwing it away after one or two uses (very cheap brushes tend to do things like starting to lose bristles as you paint which is a pain).
Also, depending on how much you plan to paint (e.g. 1 room vs the whole house/apartment), buying the primer in larger quantities (e.g. 5 gallons) can be a good cost savings (this applies to paint too if you're going with one color for the whole house). You probably don't want to tint it if you plan on using it with multiple colors, but I've haven't really seen a major need for it. Tinting can make things a little easier, but I personally like having a lot of primer on hand so I can really make sure I prep my walls as well as possible rather than worrying about running out of primer in a specific tint (so one 5-gallon can of white primer was my choice for painting 5 rooms in my house in various colors and I still have a decent bit left).
As far as paint goes, there are quite a few options, but three of the more common ones are:
Flat - It's your standard issue paint in areas that don't see a lot of moisture or prolonged dampness. However, it does have a tendency to get dirty easier than other alternatives.
Satin - Is my preferred paint for areas that don't see a lot of moisture. Unlike flat paint, satin has a slight (hence satiny) sheen to it and it looks good without looking outright shiny like semi-gloss. This also means it tends to resist dirt and grime better than flat paint.
Semi-Gloss - This is what you should use in your kitchens and baths. It's got a good bit of shine to it and it tends to handle moisture and humidity better than satin or flat so it's easier to keep clean (you can wipe it down as needed). If you have kids, this is also an option in areas that see a lot of hand-to-wall contact like a hallway or a playroom.
Once you know what kind of paint you want and you've settled on a brand, you can go ahead and choose a color. Some people might say that you should pick your color before the brand, but in my experience and given the color selection for even the least expensive paints, you're better off keeping you budget in mind and then working within what you can afford.
I'm not even going to try and suggest how you go about picking a specific color, but if you're very picky about shades and slight differences in colors, don't rely on a the small paint chips you find at the store. Buy a small sample can (or cans if you want to try multiple shades/colors) and paint it onto something fairly large (a piece of canvas or piece of plywood or sheetrock, whatever is convenient for you.. oh and don't forget to prime it too). Then put it in your room and see how it looks at various times of day and see how it looks against your furniture etc... Make sure you really like it before you proceed. This step isn't absolutely crucial, but it's certainly one to consider before you jump in and start painting.
Also keep in mind that the actual colors and their lightness or darkness can significantly impact your mood. Do a little research into how colors affect mood, and consider how you and your other household members feel about the colors you're considering (some people are more affected than others). The last thing you want is to make a space unpleasant to be in by choosing a color that is not conducive to the room's activity (e.g. concentrating in an office, relaxing and resting in a bedroom, etc...).