Log into your account at https://bittrex.com/, go to "Settings" in the upper right, and select "API Keys".

Click on "Add New Key":

If you intend to use the service for trading, then turn on all permissions except "WITHDRAW", as in the screenshot below.
If you only need the service for infographics, then turn on "READ INFO" only.

Enter the 2FA code and click on "Save".
And now the API & Secret keys are in your hands: 

 If you need to delete the key for some reason, click on "Delete", enter the 2FA code and press "Save".

Enabling 2FA 

Two-factor authentication (2FA) is used to improve account security.  When you log into Bittrex, in addition to your password, you will enter a six-digit code that will be generated on your mobile device. To use this function, install the Google Authenticator or Authy app (we recommend Authy). 

As soon as you install the app, go to your Bittrex account settings and select "Two-Factor Authentication":

Open the app on your phone, find the add account option and scan the QR code provided on Bittrex (you can enter the Secret Key manually, but this will be less convenient).

Important: write down your Secret Key in a secure place. It can be used to recover a lost 2FA. And for this reason, never divulge it to anyone.

 As soon as you add the account to the app, a random 6-digit code will be generated every 30 seconds; enter it in the "Authenticator Code" field and press "Enable 2FA". From now on you will now enter this same code for authorization on the exchange. 

Please note: each code is valid only once until it is entered correctly the first time. Thus, if you already entered it and you need to enter the 2FA code again for some reason, wait for a new code to generate; otherwise, you will get an error.

Now open the email account you used to register on Bittrex, find an email with the subject line reading "Bittrex Two-Factor Authentication" and follow the link inside. If there is no email, wait 5-10 minutes and refresh the mailbox.

A page will open where you will be asked to enter the 2FA code again; enter it:

Press the red button "Enable Two-Factor Authentication" and voila – you have enabled 2FA on your account.