I forgot a form

Since you have already e-filed or mailed the return, you will need to wait until the IRS has accepted or rejected it. Once they accept it and you have received your refund, then you can amend it to add that 1098-T. If it gets rejected, you can just make the needed changes and file again. 

Once you receive your refund, you can go here https://ttlc.intuit.com/questions/3061514How do I Amend (Change or Correct) a 2015 Return in TurboTax? and get instructions for amending. 

And the instructions are here https://ttlc.intuit.com/questions/2562661 for where to enter the Form 1098-T when you are amending the return.