The university's own website has a fantastic guide to the application process. Have a read on the link above for all the information you could ever want about how to apply to Oxford. What we would add to the university's website is that you really shouldn't be shy about applying. We're not crazy super geniuses (genii?) at Oxford, just normal people like you, many of whom applied just on the off-chance of an interview.
Applying to Trinity
You can apply to Trinity in exactly the same way you'd apply to any other college in Oxford. The key difference is that Trinity - as one of Oxford's smaller colleges - does not take applicants for every subject. Take a look below for a list of which subjects Trinity currently offers. We really like this feature: it makes Trinity such a close-knit community where you can easily know everyone else in college within a couple of terms, by virtue of having a smaller number of students.
If you have any questions about the application process, or life at Trinity, get in touch with the JCR's Access Rep.