In the UK you will apply for university through UCAS and you should look to their website for information on that process; however, to make your application as strong as it could be there is some good advice that you should follow:
It may seem obvious, but you should fully research every course that you apply to. You should know what it involves and what opportunities there may be within the course. Some courses offer work placements which can help provide experience, others offer time abroad. It is also the case that a course at one university won't be the same as one with the same name at another university, so once you've chosen what you want to do and where, you shouldn't just throw in another 4 similar courses without checking. Remember that your 5th choice could end up being the course you go on to do, so do your homework and know what it involves. It really is worth repeating that you can't rest all of your hopes on your first choice, and should make sure that the other courses you include in your application are courses that you want to do, otherwise don't bother adding them.
When you are applying to university, the onus is on you to manage your deadlines and to know what is involved. You will need to meet the UCAS application date, so make sure you know what that is, but part of the process is to get a school reference and other paperwork and all of that is due at the same time. You are responsible for your application, not your school, so you need to be ahead of the deadlines and on top of what has been done and what still needs to be completed. It is better to have your own deadline well ahead of the real one, and to share your progress with whoever is going to be writing your school reference, so that they know when you need that done. You should be fuly aware of every step of the process, and all of that is available on the UCAS website; there are no secrets or hidden steps, it is up to you to research all of this, and given that it will form the next three or four years of your life as well as shape your future career, this is an essential step.
Your personal statement should be about you and your suitability for the course you have applied for. Avoid long quotes and other cliches and stick to what makes you right for your chosen course. Hopefully your chosen courses are all similar and your statement works for all of them, but sometimes people apply for quite different courses and that makes your statement more challenging, in which case you should emphasise your skills and attributes. Generally, though, the five selections will all have something in common which allows you to be much more specific in your statement. You should show some evidence that you have been working towards this course; universities like to see that you have given your courses some thought and that you know what is involved, so research is important. The Super-Curriculum is an important part of any excellent application, so if you are looking for a strong application you should have a track record of experiences in the field that you can talk about. Competitions, events, podcasts, books and many other activities can improve your knowledge and make you a better, more informed, candidate, so check out our subject pages for ideas on what you could do.
Applying to university is one of the most important things that you will do in your school career, and just like exam coursework, difficult personal situations or other difficulties in life, you should speak up as soon as there is an issue or problem. If you aren't clear on the process, if you don't understand what a form is asking you, if you can't express what you are trying to say in a personal statement or you can't quite work out how the course you are interested in really works, then help is always at hand. Ask a teacher if it is an issue that they can help with, but don't be afraid to contact the university. The person who handles your query is not going to be the person reading your application or making the decision, so don't think that asking a question about the form or entry requirements is going to somehow impact your application, if anything it shows a bit of character and that you are interested, but it will make no difference to how the university judge your application. Your application should always be moving, if something is blocking or stopping your progress then you need to act on it immediately and not wait months for the answer to materialise. You will miss your deadlines, rush your choices, or make an error if you don't address whatever is troubling you, and there are people that you could and absolutely should turn to for advice.
When you go to university, you will spend a great deal of time in that location, so it is important that you have considered your personal circumstances. Will there be a cost to your tuition and how will you fund that? Do you like the city that you are applying to study in; have you been to spend some time in it to find out if you are going to enoy living there? If you are leaving home for university, have you thought about the cost of that, and worked out the best way to fund that? Often people have the chance to stay at home for university (if you live in a big city) but move out to pursue their independence or to have the "university lifestyle." Make sure you know what you want, and what works best for your circumstances; you don't want to be one month into a course and realise that you hate the city, or that you can't afford to stay there because you presumed that you would get a part-time job straight away and can't or you don't have time for a job because the course is so much busier than you realised. Personal and family circumstances are an important consideration, and part of your wider university research - make sure that when you have applied for a course you have really done your homework and know exactly what it will involve, inside and outside of university.
It's easier said that done, but worrying never helped anyone, and once you have applied all you can do is wait to hear back. Don't impatiently contact the university because a friend has had an offer and you haven't, they have your application and are working through their list - not always in the order they were received either. Focus on having a good year whilst you wait, it might be that you must meet the universities conditions and you need to be focused and ready to achieve that in exams. Even if you have no conditions, and are accepted to a course, you should use your time wisely to prepare for your course, by keeping up super-curricular reading and activities. Geting into university is only the start of the next stage, so you have to be keeping up you record of achievements so that you can apply to jobs, take on leadership roles at university or bag a place on a competitive internship halfway through your course. If the application doesn't go your way, or you fail to meet your conditions, then your life is not over, and worrying is not going to help. You can use the UCAS clearing service to find a course that will take you, or you can make alternative plans for a year and apply again. In that case, more super-curricular experiences are essential as new candidates will be against you the following year and you need a way of standing out rather than a one year gap in your CV. Whatever happens, you should remember the advice about speaking to someone if you need to, either about your application, or about your mental health if you are finding the process tough or worrying. It is natural to worry about leaving the safety and familiarity of school and going to a new place, potentially many miles from home, so don't think that this is unique to you, and don't hestitate to reach out for help.