First of all, You will need to get into your admin page and hover your mouse over the menu item 'settings' and then select 'commerce'. You will then scroll all the way to the bottom of that page and the last thing you will see is a box to check that says 'Gift Card' that you will select and enable. Click 'Save' to save your changes.
Now you will go back to the homepage of your admin and hover your mouse over 'Shop' and select 'Gift Cards' in the drop down options.
Once there, You will see 'add gift card' in the upper right hand corner. You will then be able to click that and add a new gift card. This step is crucial, if you dont add the gift card in here first, your customer will not be able to utilize the gift card as it doesnt exist in the system.
It will auto generate a code for you but you may also insert one if you please, then you will add the currency type, value of the gift card and when it will expire (if it does). You will then save the changes and add it to the system.
You have now created and activated a gift card that your customers can use online to purchase with. The system will automatically keep total of the balance on the card before and after purchases are made with it, so you will not have to worry about that yourself.