Your school should provide an essential source of advice on that next step but remember that this may be driven by one or more of several motives. Primarily should come doing the best for each student, irrespective of where they continue their studies. Secondly, there will often be a strong financial motive for encouraging pupils to stay on into the school’s own sixth form where there is one, and thirdly schools may wish to be seen as academically successful encouraging and setting admission rules to select the highest performers at GCSE.
A few other reliable sources of information and advice are listed below, but some who seek to advise you will come with pre-formed opinions such as the relative value of academic and vocational qualifications. Most schools arranged Open Days for potential new students in past years, often before Christmas, but in this exceptional year it may be much more difficult to visit and you will need to work harder to find out details. One key piece of information is the necessary GCSE grades required to enter the institution, but be careful as individual courses often require specific levels. Another to which you are entitled but too often is not readily provided- so be persistent - is the number of students completing relevant courses and the results obtained. For some courses, drop-out rates are strong indicators. Progress Grades are the most important measure for A Level courses. For schools, my website provides some details of 2019 A-Level results in the Individual Schools section or go to the Department for Education website below for more detailed information (results for 2020 will not be independently published).
Some popular schools have early closing dates for Sixth Form courses, including at least one in January. Some make provisional offers based on forecast grades, which I have long maintained are unlawful!
If you are choosing a course with a particular degree or career in mind, you should check the requirements before selecting subjects. If you are struggling with any of these at GCSE level, then perhaps you need an alternative career plan. My own experience is that many students change their ambitions during the sixth form, so there is an argument to simply pick your best and favourite subjects, although there are some obvious pairings such as maths should always accompany physics.
And finally, remember it is your future that will be partly defined by the next step; other people’s opinions are just that!
CXK: https://www.cxk.org/services/career-guidance-young-people/
National Careers Service: https://nationalcareers.service.gov.uk/
Kent County Council: https://www.kent.gov.uk/education-and-children/college-sixth-form-employment-and-training/choices-at-16-and-beyond
Department for Education Performance Tables: https://www.compare-school-performance.service.gov.uk/