I would recommend considering the subjects you have enjoyed thus far in school and/or college and start from there. Once you have got your list, look through different university prospectuses and what they have to offer. For example, if you are interested in sociology, then which aspect of sociology do you enjoy most, what is it about that specific area, etc. The prospectus let you see the modules on offer at university and you can also attend their open days if you wanted to directly speak to the lecturers themselves.
If there’s a particular course that you think you’re interested in right now, have a look around different unis and see what they offer in relation to that course. Open days are a really good chance to do this as well as speaking to people who currently do the course you might be interested in to see how they feel. Even if you apply now and change your mind in a couple of months time, then you can always withdraw your application if you think that uni might now be for you, or you can change your entry date to 2020 rather than 2019 so you have a gap year to do something different before continuing with your education.