If you are using the Online version, you can add your spouse's information in My Info. You will select Edit next to your filing status (it's okay that it says Single for now), and then change it to Married. TurboTax will ask you if you want to file jointly with your spouse and then gather the required personal information. 

If you are using the Desktop version, you can add your spouse's information in Personal Info, select Edit next to your filing status and check the box that says Change my filing status. Select Married Filing Jointly and it will then ask you to provide your spouse's personal information.