If you have the code for the online test/quiz, you can enter it by clicking the "Join" button on the homepage. If you don't have a code and the test/quiz is public, you can find it using the search option. If it's private, the creator needs to invite you first, and you can join it from the "Invited" option on the homepage. If it's a paid test/quiz, you'll need to pay the price set by the creator before you can join, whether you found it by code, search, or invitation.