first open steam than enter the steam store there will be a box that says search the store

put a game name in there
go to the game page and click add to cart
How to buy a game on steam-add-cart-png
than you will be redirected to this page
How to buy a game on steam-steam-shopping-cart-png
click purchase for my self or if your gifting a friend click purchase as gift
after that youle be on this page

How to buy a game on steam-purchaseing-jpg
leave it like that if your useing steam wallet
if your not useing steam wallet you can click on that and select a differant payment
How to buy a game on steam-differant-payments-jpg
the rest explains its self enjoy