Career in Acting and Writing?

It looks like you're in the UK, so you're going to need high grades at a lot of GCSEs and then very good A Levels.  After that, you'll have to win a place at a top drama school, such as RADA or Guildhall.  They're very expensive and all of them get over 6,000 people auditioning for only 24 places every year.  They only take a few of the very best.  

To stand a chance you'd need to have studied with a really good acting class/school at the weekends or evenings for several years and have been performing regularly with a reputable youth or community theatre company for a long time.  You'd also have to prove that you can afford to pay the fees in advance for three years, as well pay to live nearby (usually in central London) and afford rent, bills, food, clothes, transport, etc.You'd have to be in excellent health, very fit and, of course, extremely talented.But you need to understand that it's incredibly rare for any actor to work full-time.  The vast majority of actors never get more than a week or two's paid work a year.  The rest of the time they work in an ordinary job and fit in the occasional day or so's acting where they can.  Also, very few people make any money from writing.  You're going to need to get qualified for a well-paid, interesting 'day-job'.  Anything will do as long as you could take time off whenever you need to for acting work, rehearsals, masterclasses, line-learning, auditioning, etc, so being self-employed would help a lot!I hope that helps.  Good luck!