To add VAT to your invoices you will need to enter your VAT number in your company settings.
Open your "Company Settings".
Enter your VAT number in the top right corner.
Click "Save" in the bottom right corner.
If you need to charge for VAT, but have yet to receive your VAT number, you can activate VAT on your invoicing by simply entering the text "Awaiting Number" where your VAT number should be in the above picture. Remember to add your correct VAT number when you do receive it.