If you don't want/don't have time/can't install a special program for making screenshots, like ShareX, you can do it with Windows' system tools.
MS Paint (XP/Vista/7/
To open it:
Start –> Programs –> Accesories –> Paint
Start –> Run –> mspaint
Win+R –> mspaint
When you want to make a screenshot, press PrintScrn (to make a screenshot of the whole screen), or Alt+PrintScrn (to make a screenshot of the active window).
Then go to Paint, press Ctrl+V (or Edit –> Paste) and then, File –> Save As....
It is an example that i have take from Tanki Online game.
Snipping Tool (Vista/7/
To open it:
Start –> Programs –> Accesories –> Snipping Tool
Start –> Run –> snippingtool
Win+R –> snippingtool
With Snipping Tool you can make screenshots of the whole screen, aswell as of a part of it.
To select a 'shape', click on the arrow, near the button New and choose the desired option from the dropdown menu.
Marker and Pen tools allow you to underline, add notes and stuff, if necessary. Eraser tool allows you to remove objects.
Screenshots in Windows 8 (alternative way)
In Windows 8, you can make screenshots by simply clicking Win+PrintScrn.
This key combo makes a screenshot of the current screen and saves it as a PNG image in the pictures folder by default, i.e. the folder Pictures (C:\Users\nameofuser\Pictures\Screenshots).
Every file automatically gets a name, like 'Screenshot (1).png', 'Screenshot (2).png', 'Screenshot (3).png', etc..