You have to hold a valid passport to enter or leave the UK. You may also need a visa to travel to other countries.
If you are travelling to another country from the UK, your passport may need to be valid for your whole trip. You should renew your passport before you travel if it is due to expire while you are away.
You may also have to attend an interview as part of the process of applying for a passport.
All new, full UK passports use facial biometrics. Your passport photographs will have to meet a required standard set by the International Civil Aviation Organization.
'How to apply for a UK passport' sets out the passport application process and what you need to do. 'Applying for a UK passport' explains specifically what you must do to apply for a UK passport in the country you are in. You need to read both pages to have all the information that you need to apply for your passport.