In Windows, you have two options. Press the Print Screen (or PrtScn) key on your keyboard to copy the entire screen to the clipboard. Press Alt-Print Screen to copy only the active window. Next you'll need to open an image editor (Microsoft Paint, which comes with Windows, is fine if you don't plan on doing any fancy editing), create a new file, and select Paste from the Edit menu.

You have more choices on a Macintosh. Pressing Command-Shift-3 creates a PICT file of the entire screen in the top directory of your boot drive. Press Command-Shift-4 to change the cursor to a cross, which you can click and drag to select an area of the screen to capture in a PICT file. The same combination with Caps Lock activated changes the cursor to a circle; click a window with the circle to create a PICT of that window. Adding the Control key to any of these combinations copies the screen image to the clipboard.

For X Window desktops such as GNOME and KDE, the command xwd -out filename.xwd lets you click a window to make an XWD image of it. Adding the -frame option will capture the entire window manager. The popular utility xv also does screen captures, and KDE comes with the utility ksnapshot.

Finally, if you plan to edit the screenshot before using it on your Web site or elsewhere, be sure to save it in a true image format. Don't make the screenshot endure GIF reduction or JPEG encoding twice.

Then if you want upload the image to a hosting company such as http://photobucket.com/ which is free.

Then to post the image in a forum surround the link with img codes like below.

For Windoze users, Irfanview is a free download offering quick and easy scalability for enlarging and reducing screenshots and other images.  The most recent version also supports animated gifs.  Screenshot pasties are done the same way as in Windoze Paint.

windows vista has a snipping tool that works really good