I think it's really important to realise how much a subject demands of you and make sure you do your work for that subject, and realise pretty early on if you want to keep that subject or do another one instead.
Don't procrastinate, find that drive to get the work done at a designated time so that it doesn't all pile up.
I guess the most important part is, find it within you to enjoy that subject, and let friendships grow naturally. Don't get too upset if things don't work out, and don't seek a relationship - if one happens, it'll flow naturally.
From GCSE to A level, it's all changing