Well, first you need to download Steam. Now, you can either get Steam with the cache files, meaning that Steam will not have to download the cache files from the server, or just get Steam by itself. I recommend getting Steam by itself since the cache versions are larger in file size. Once you download Steam, you need to make a Steam account. From there, you can buy Half-Life or any other Steam game off of the Steam Store, and it will be added to your account.