It's kind of a weird thing until you finally figure out what's going on. Kind of like going down the highway wondering why everybody is going in the wrong direction...
When first starting out using photoshop and well other photo editors for that matter I thought I was pretty good. I could adjust just about everything and images turn tack sharp, colors and brightness/contrast was just spot on.
First weird incident was when printing they would usually come out too dark. Of course this would have to be because it wasn't the right paper or you know it's a great printer and all but after all it's not an Epson which is know to be the preferred photographer printer.
That thing I solved by increasing the brightness before printing so it looked too bright but the prints came out in a decent way. Another thing to "solve" it is to let the printer handle colors and brightness/contrast and tada problem solved even though you loose control of the output (well somewhat anyway).
Next up. Posting photos on webpages, forums, blog etc. It still looked great on my computer but when I went to work it just didn't look like I saw it at home. Hmmmm must be those cheap monitors at work that just cost twice of the one I am using at home...
Of course it turns out that my monitor at home isn't calibrated correct or well no where near where it should be and because of that I could manipulate the photo to look great on my screen while it would in reality (judging the bits) look off in colors, brightness and contrast.
Starting to read up on it (a while ago) I was thinking that buying a monitor calibration device would be the easy way and it sure is. And while we are at it "just" add a print calibration tool as well...
As you probably guessed this doesn't come cheap and have a price tag that well... yeah if it's your living go for it. If it's "just" a hobby maybe there are other items that would be first in line.
A very simple step to get close to the correct calibration is adjusting the brightness and contrast of your monitor using this gray scale strip.
(note: if you are using a crt monitor it should be warmed up before adjusting. To put it simply have it on for 30 min before tuning the display.)
This test strip shows a range of grays from pure black to pure white. You should be able to see a difference between each shade of gray with pure black on the left and pure white on the right.
This may not be a perfect adjustment but as mention. It's kind of close and should be enough to avoid ooops moments when posting online.
Along the top of the strip are alternate patches of black and dark gray. If it looks solid black to you your screens brightness setting is too low. Increase the brightness until you can (just as in barely) perceive the difference between the gray and black squares.
When setting the brightness/contrast you will have to ensure that you can see each shade of gray in the range without any of the merging or looking to be the same shade. It can be a little tricky but doable.
If the adjustment makes the strip look like either of these two strips below it is set too bright or too dark...
ohh and when adjusting the screen you may want to
1) Set the contrast of your monitor to its maximum (usually or at least set it high).
2) Reduce the room lighting and try to avoid reflections in the monitor.
3) Set the monitor to display "millions of colors" and have it switched on for at least 30 minutes.
As I mentioned this is far from the best method out there. But just a quick little write on one way of getting close to a calibrated monitor.
You may also want to check out this blog post that goes more in depth with calibration and shows different approaches including Adobe Gamma Tool (found in the control panel if you have photoshop) and touches on hardware calibration.