So you want to maximise your chances for getting into university. Well you're in luck! If you get started at the end of Year 12, or even before, you'll have everything you could possibly need to maximise your chances.
Things you can't change
There are parts of your application that have already been decided. In essence, these are things you can't really change, and you'll have to make the most of what you can change later on.
In the perfect world, your GCSE grades would be 10+ grade 9s. Now, given that there are only around 600 or so people who have achieved this in the last year, it's quite possible that you, the reader, are not one of them. Don't let this discourage you! While your GCSEs play a part, they don't play as massive a role as, say, A-Levels. Many universities also view 8s and 9s as equals. The main university that seems to value GCSEs more than average is Oxford. If you're applying to other universities, then this shouldn't be to big of an issue.
Another aspect of your application that plays a role is your extenuating circumstances. These are things that may have uniquely affected your education such as illness and personal matters. Universities will consider your application using this context if you apply for an extenuating circumstances form.
Finally, the last aspect is the school you go to. Given two identical students, universities will likely prefer the student which went to a worse school. This is to balance the lack of opportunities compared to students from those in better schools. So, if you happen to go to a better school, universities will cut you less slack.
Things you can change
These outline the things you can change about your application right now. The year from Year 12 to 13 is the most important year of your application, as this is when a lot of the major parts of your applicaiton can be changed.
Your academic performance plays a decent role in the universities' eyes. At the top end of the spectrum, most students applying to competitive universities will likely be predicted 3A*+. If possible, try and reason with your teachers to improve your predicted grades to match this standard. Taking more A-Levels doesn't necessarily improve your chances. In fact, it's possible that universities require you to get minimum grades in your 3 subjects AND your additional subject(s). An exception to this is when you're applying to mathematical subjects, where a large majority of applicants will be taking Further Maths as a fourth A-Level. You should do too.
The personal statement is another important factor of your application. This is where you can fit in all your activities and demonstrate your ability to go above and beyond in your subject. Though I will write about the personal statement in more depth in following articles, the basics you need to focus on are including actions (things you have made, produced, or done) which link back to your course, and to have a maximum of 20% of your personal statement talking about extracurriculars (activities outside of your subject. I personally didn't have a bombshell of an opening line as I find those statements a little cliche. At this moment of time, at early March, you should be trying to maximise the number of actions (supercurriculars) that you can include in your personal statement.
Universities are likely to read your references too. These are messages from your teachers which should really sell you to the universities. Teachers should take note of any awards and achievements you've made within the school related to your course. Think of it as a free extension of your personal statement.
In the same vein as academic performance, your admission test score also plays a role. Univerisities may ask you to sit an admissions test for your subject, just to confirm your aptitude beyond just having the grades. I personally would recommend at least 2 months of revision for most admissions tests. I was able to obtain top 1% scores on 2 different medical admissions tests in this timeframe. This (I believe) contributed the most to my Cambridge offer.
The final aspect of your application is your interview. In highly selective universities and courses, you may be asked to interview in front of representatives of your course. This will test how suitable you are for both their course and their university, and plays a significant factor in your applicaiton. This was one of the weakest parts of my application, and in the following articles, I'll explain the mistakes I've made and what I would've done differently if I had the chance to interview again.
Summary
There are many parts of your application that play a part in whether you get offers from univerisities or not. While you can't change your circumstances or GCSE grades, you can be improving your academic performance and activities for your personal statement right now. Good luck.