Your college or school will help you on how to apply through UCAS. It is a standard on-line form which you should start filling in at the start of your final year. Be aware that there are important deadlines in the application process and you must adhere to them; UCAS will provide all the information as to exactly when the deadlines are.
If you are successful, you will be invited either to an open day and/or an interview where your application will be discussed. You should ideally go to any open days to find out more about the course and university to see if it is right for you. Take the opportunity to ask them questions too at your interview.
If you are not successful, UCAS will notify you. You may able to receive feedback on why by contacting the Undergraduate Admissions Team.
A firm or conditional offer of admission can be made before or after each interview/open day from your chosen univerisites. It will detail the grades required in order to be accepted onto the course. When you have received them all, you must decide which you are going to hold and which you are going to decline. This information has to be transmitted back to UCAS, and UCAS will tell you exactly how to do this and will also make you aware of the commitments you are entering into.
If you meet the entry requirements, then you will be admitted. the university will be in touch.
If you do not meet the entry requirements, there is still a chance you can be accepted. If your grades are only slightly below what was expected, contact the Undergraduate Admissions Team directly to see if they would still consider you. Otherwise, UCAS will invite you to enter the clearing system. This is a very hectic period when UCAS endeavours to match applicants to vacancies. If you find yourself in this category, you must be available to react very quickly to any offers that are made and to make your decisions very quickly. You might not be able to gain admission to a course or university that ideally suits you; you must be in a position to decide whether what is on offer is sufficiently attractive.