Things You'll Need
Instructions
Download and install Steam for Macintosh. Visit steampowered.com and click the "Install Steam Now" button to download a Steam disk image.
Double-click the .dmg file to mount the disk image. Drag and drop the Steam installer to your Applications folder. Once copied, eject the disk image by right-clicking on it and selecting "Eject." You can then drag the .dmg file to "Trash" and empty it to free up hard drive space.
Launch Steam by double-clicking the application stored in the "Applications" folder. When you first launch Steam, the client downloads the Steam software and necessary updates. The application you copied to the "Applications" folder in Step 2 is a small download that permits this larger installer to run.
Create a Steam account, or log in to your existing one. Click the "Join Steam" button in the Mac OS X client, or visit store.steampowered.com/login and click on "Join Steam." Complete the form to register, and click "Create My Account" when complete. If you already have a Steam account, log in using your exiting credentials.
Review the Steam homepage. After logging in, you are redirected to the Steam home page. Click on the "Mac" tab near the top of the screen to view the Steam games compatible with Macintosh. Click on a game title to load its page and purchase it, if you desire. When purchasing, you will be asked for payment information before the download commences.