After you Minecraft (Mojang ID), you can redeem it and download game follow the steps below.
1. Create an account at account.mojang.com/register
2. Check your email for a verification email from Mojang. Follow the link in the message to verify your email address.
3. After you have registered a Mojang account, you can redeem your prepaid card or gift code at account.mojang.com/redeem. If you have a card, scratch off the silver area, enter the ten digit PIN in the field under "Enter Your Gift Code or Prepaid Card PIN", then click "Redeem".
4. Choose your Minecraft username. You can check if a name is available by typing a name in the "Profile name" field, then click "Check availability". If the username is available, accept the terms by checking the box next to "I have read and agree to the Minecraft End User License Agreement", then click "Choose username".
5. To download the game, click the Windows, Mac or Linux icon on your account page at account.mojang.com/me. On a Windows PC, download the launcher, place it anywhere you'd like, then run the program. On a Mac, download the file, unpack it, then place Minecraft in your /Applications folder. You can download the game on as many computers as you'd like, as many times as you'd like.