Run Steam in the desktop and enter your username and password to access. After you download the utility, go to the tab "Friends" - "Add a friend" in the upper toolbar.
The application will require you to enter the nickname of the user you want to add, or the name specified during the registration. Best search options to specify the username of the other, because search in game nick is not often successful due to the large number of registered users. In the results list, select the appropriate position and send the request to add to your contact list.
If you have not yet installed any games on Steam, when you try finding friends you will be displayed a message about the need to buy any app in the store. In this case, you will be able to add the needed person through an account on the official Steam website. Open a browser and go to tools Steamcommunity. If necessary, click on the "Login" in the upper right of the screen and enter your username and password to login.
Then go to "Your account" - "Profile". Copy the address of the loaded page from the top row of the browser, right-clicking and selecting "Copy". This address is the link to your Steam account.
Contact your friend and send the copied link to your profile. Friend will need to paste the link into the address bar of the browser to log into your account, and then choose on your profile page the link "Add to friends". After this friend's nickname will be displayed on the control panel website, and in the appropriate menu of the program.
To approve a request to add friends from other users, in the Steam client go to "Community" - the "Invitation". You can also use the "Friends" tab to connect with other contacts and see their online status.