You need to create a Sourceforge account. Go to Sourceforge site, click Create account on the top right corner and fill in your personal information. After few minutes, you'll receive a confirmation email with information how to activate your account.

Your group should also create a new project in Sourceforge. One of the members of your group logs in and creates the project - he or she will be the admin of the project by default. Add other members of your group as developers for the project.

After the project has been created enter additional information about the project, like which operating system your application is targeted for, who is the project aimed for, etc.

Then it is time to either start programming, or perhaps you could copy an existing project and save that in Sourceforge as a fork of the original work.