There's a few different ways of doing this.
You can download a screenshot utility that will automatically save your screenshots into a folder. I personally use
PrintScreen by Gadwin. This is especially useful if you intend on taking action shots during events or pvp battles.
If you don't want to download any type of utility there's still hope for you.
1- Press "Print Screen" on your keyboard
2- Open up Microsoft's Paint program (most likely located in your Accessories tab on the Start Menu)
3- Hit "Edit" in the menu bar along the top then "Paste" in the drop down (or press CTRL and V on your keyboard for the shortcut). Your screenshot will be pasted into the program.
4- Save the image as a .jpg file. ("File" then "Save As". When the pop up menu to save comes up, hit the drop down for "Save as type" and select JPEG).
As for posting images, Stratics allows you to attach images to your posts so you can directly upload your image from your harddrive to Stratics. If you don't like that idea, you can sign up for a website that offers image storage (I use
Photobucket). To use this method, copy the IMG location from the site once you've uploaded your image and then paste that url into your post.