You should've gotten an email from Autodesk asking you to activate your BIM 360 Design contract. if you didn't or you don't know go to the BIM 360 Activation site https://bim360.autodesk.com/activate/c4r-docs-account/ and fill out the form. Check the box to agree and click Activate Account.
You will then see that you have successfully activated your account. From here you will need to wait for the email that will come from Autodesk and click on the blue button "Activate your account". This will take you to the BIM 360 site were you will sign in with your Autodesk user name and password.