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.

I agree with Cow about the sticky.    I find everytime I want to do a screenshot i have to look up (how to do). this is something every one should read 

thanks for the info

OK I got paint up and  followed the instruction's but
can t figure out how to get the screen shot from the paint
on to here.How do you transfer the screen shot onto one off these
post.

i still can't get a screenshot.  i have tried and tried.  i just gotta get this down.  it is very frustrating.  i'm about to just give up.  i have tried most of your suggestions but still nothing is working for me....

this is rough bro no offense but where just poker players. i love computers but damn. can i just take a pic of it or something?

You make look as if it is easy!  But the people I run across I tell them there are two big steps to look at!  Step 1 is saving image to your hard drive  and step 2 is bringing to the web!

I tell them the Print Screen button is the 4th button to the left of top right hand corner of keyboard!  I also tell them you can paste in MS Word, MS Excel, Power Point and Word Pad, just remember to save this as a image or picture and not as a document.

Do not mean to offend but most people are in a panic when they screw up and if you expand description it makes them read and think clearly Baybee_Dollz !