What did I need to take to the embassy for my interview
The UK does not interview visa applicants. As part of your online application, you book an appointment at a visa application centre to have your fingerprints and photograph (known as ‘biometric information’) taken. After submitting your online application, you receive a checklist confirming the appointment and telling you what to do next. You have to take the checklist, your passport, and any documents that you have ticked in the checklist with you when you attend the biometric appointment.
There is a guide to supporting documents here https://assets.publishing.service.gov.uk/government/uploads/system/uploads/attachment_data/file/549692/Visitor_Supporting_Documents_Guide_-_English_version.pdf If your girlfriend is sponsoring your trip financially you will need to submit proof she is able to do that eg her bank statements, pay slips.