Here's the 411:
You need to be a first-year applicant. If you are a transfer, you don't invite your teachers. You just print out the forms and they get mailed to your colleges.
1. Go to the 'Common Questions for Applicants' page. Read it. All. And watch the movie. You'll laugh. You'll cry. You'll be enlightened about how the Common App works. Find enlightenment here: https://www.commonapp.org/CommonApp/FAQ.aspx
2. Fill in your high school information on the Academics section of the online application.
3. Go to the School Forms page. Read the instructions.
4a. If you high school is configured to submit school forms via Naviance (the instructions will tell you if this is the case) then you need to talk to your counselor and teachers and they will submit the forms electronically via Naviance. You won't be able to invite your teachers from the Common App Online.
4b. If you high school does not use Naviance, you'll see instructions telling you how to invite your teachers and counselor.
5. Still confused? Take a deep breath and re-read the instructions. Everything you need to get started is there. Search CC as a last resort. There's a lot of information in these forums, but it's not all correct or easy to understand.