You can add your advanced passenger information (API) when you check-in online. This is where you enter in your passport number, DOB, passport issue and expiry date for example. A few airlines we use do not have online check-in facilities. Don't worry the agents at the airport will do this when they check you in.
You may also find this article helpful, there are some links to airlines websites we use. Do I need to check-in online for my flight?