First you want to open up Steam and then log in.

Now that you are in Steam look on the left hand side and this thing called "Libary" will be there now i want you to look for a thing called "Polygon Attack" or any game you want to play.
Note: you may need to use the scroll wheel in order to see more games.

Now that you have found the game you want to start you will now have to click on the button at the top near the game name that reads "Play"

Disclaimer: Should you in case select the wrong button and end up doing something else to the game I am not responsible and you shoud reread this guild and then try one more time if that does not work then you are on your own good luck.

Thats all there is too it have fun with the game you play.