If you're thinking about applying to Emmanuel, then you'll need to first of all complete your UCAS form and send it to UCAS by their deadline. After your chosen College has received your application you will need to fill in a Supplementary Application Questionnaire (SAQ), and there may be other demands. Most interviews take place in December.
For a full explanation of the applications process you should visit the University's webpage about it here.
Depending on your subject, you may be asked to send in an example of written work or sit a test (what happens may change each year). For example, Geographers were asked to send in two examples of written work and were sent an article to read and think about for discussion in the interview.
You'll probably have two interviews, one tends to be more social - about your motivations and personal interests, and one is more academic.