Do I have to buy another subscription in order to create forms when I already own the full creative suite?
Sorry, FormsCentral is not part of Creative Suite. You can try the service with a free FormsCentral account but you will have to subscribe to a paid account to access all FormsCentral features.
That depends on exactly what you mean by "create forms" and which version of Creative Suite you have. If you have Acrobat, you can create PDF forms that can be used with FormsCentral. Creative Cloud includes Acrobat Pro XI, which also includes the FormsCentral desktop application which will create PDF forms as well.