Megapixels
Nowadays, the question of megapixels (MP) is almost moot. Almost all basic DSLRs out right now have at least 10 megapixel digital sensors. Megapixels are merely the total pixels or points of light that a digital sensor has to make an image. Having more does not make for a better image, it only allows for that image to be blown up to a larger size. But 10MP is sufficient for almost any size print you are likely to make
these days.
To figure out how big is pretty easy. In the specifications for your camera you will see a Pixel x Pixel for a large image specification. It will look something like this from a Canon Rebel XSi - 12MP - 4272 x 2848. Take those pixel dimensions and divide by 300 which would give you 14.24 X 9.49.
That is the maximum print size in inches for a Museum Quality Fine Art Print for that camera. Is that the maximum print size for that camera? No, not at all, in fact you could print up to a 42" x 28" print from that camera with very good quality which is
large enough for just about anybody's wall.
So you can see if you just are going to print 4" X 6" or 8" x 10"s every camera out there is sufficient for your needs. In fact if you only email or post pics to your Facebook account, those images are under 1 MP so there is no need to worry at all. Since you are here to learn and get better at your photography, we would hope that you would like to print some nice 11 x 14's to display on your wall!
In some cases, too many megapixels may actually be a hindrance. If you don't have a high end computer with a fast processor and plenty of memory, you may not even be able to process an image from that 21.2MP behemoth you are ogling at the photo shop. Plus, too many MPs may actually negatively affect high ISO performance on a lesser camera. So while MPs are important, it is not the specification that should make or break your decision on a basic DSLR.