i resat year 12 (ive just finished yr 13 and am going onto uni this year) i would advise to make sure you really do as much work as you can this year - you dont want to feel like you have done an extra year for nothing! if youre resitting all the same subjects (i did 2 retakes and 2 new ones) this is the time to really make sure you understand and key concepts/ideas and really grasp the exam technique, even if you feel like its a bit repetitive or boring you still need to focus, getting the same lessons taught to you two years in a row can feel boring, however it really allows you to get a full understanding of the content - repetition is such a good way to drill the content into you brain!
apart from the studying, try and make as many friends in yr 12 this year (while still hanging out w ur friends who have moved to yr 13) so that next year you wont feel left behind or anything!