hey! in year 13, did an epq in y12 and got an a* :-)
1) the first thing to do would be to ask a member of staff is the epq is an option. if it is, then u probably have the option to take it.
2) an epq is basically a long piece of writing you make to answer a question of your choice. for example, my question was "to what extent should comic books be considered as literature?" keeping your question both open enough to research two sides of an argument is important, as well as keeping it closed enough so that you dont have too much material to cover. you spend the next few months carrying out research, taking notes from each piece of research, and using that research to make a conclusion about your question. i feel like a member of staff would be a lot better at explaining it than me, so take what i tell u with a grain of salt.
3) its a lot of work, especially if the question you choose is not a topic youre passionate about. this would make it really hard to stay motivated enough to put in work throughout the months you have, and it could even be too much work for you to handle, based on the other subjects you do. some people also may not enjoy academic writing in general, and thats a perfectly reasonable reason to drop it.
4) it depends on the uni! usually, if you have taken an epq, the uni would give you the option to lower one grade of your offer by one grade (e.g. an a*aa to an aaa or an a*ab). some unis also require that you get above a certain grade, e.g. an a, in ur epq to allow ur offer to be reduced. this entirely depends on the uni. look through their conditional offers guide and look for any mention of the epq!
hope this helps! good luck out there!