While I’d recommend that you just download and use SnapNDrag, if you want to do it the hard way, here are the Keyboard Commands for taking a screenshot in OS X.
command+shift+3 Full screenshot
command+shift+4 Crosshair to allow selection of screenshot area
command+shift+4, then spacebar Camera to capture specific screen object
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it’s actually possible to change the screenshot hotkeys (and a lot more hotkeys, too) to a less finger-straining combination in the “keyboard and mouse” panel of the system preferences. for example, i changed mine to shift-apple-space, which i find a lot more easy to push.