Not all colleges require common application. It is the most popular but check with your colleges on their sites.
If you are doing the common application online for your colleges, you should be consistent with the forms and also have the letters of recommendations done online. I'm assuming you registered to do your application online, you can find the tab on the left side that says, "School Forms". Click "Add" and fill in your teacher's information. They will get an email from common application with a username and password to log in a special link. This will lead them to fill out the recommendation online. However, there are teachers that do not know how to function a computer so there is an option in the email that they receive to "opt out" of online recommendations.
If you are doing the hard copy of the common application (the whole application), you can print out the application and recommendation forms on the main site and have your teachers fill that out. Then mail it to the colleges that use it. Good luck.