First off with the basics. Know your grammar! It's surprising how many people do not know how to properly use punctuation and sentence structure. A great place to practise these skills is right here (lol). Make sure you recognise how to structure different text types as well (as this usually accounts for a few marks). Another thing is syntax! Some people write sentences which are worded incorrectly, but do not realise. Get someone else (a high achieving English student who preferably does not gloat) to read over your writing before handing it to the teacher. Not to mention, expand your vocabulary by playing Free Rice (and fighting World Hunger at the same time =D) or reading ... the dictionary. Yes, I'm quite serious.
Try setting little goals for yourself, such as reading 5 pages of the dictionary everyday. The next day, speed read through the previous 5 pages and then start reading the next 5.
If pondering through thousand page books, wondering why you'd need to know what a 'bortsch' is (a Russian beet soup), is entirely out of your spectrum, then try reading - well - whatever good literature is available. See reading not as a duty, but rather as a form of past time; let your mind wonder. A good start is to checkout best-seller lists on book store sites such as Dymocks, A&R, Borders et al. I recommend Tale of Two Cities (Dickens), The Power of One (Courtenay, Bryce), The Merchant of Venice (Shakespeare) and Cloudstreet (Winton, Tim).
Expanding your vocabulary also has big impacts on your comprehension of various texts you may read in class. Because you have a wider word base, you are able to recognise and process more of the information you are being presented.
Participate in class discussions (if you have any), and express your ideas freely (but make sure you are not being too sporadic, and can easily support your opinion, otherwise you may get burned).
Finally make sure you ask your teacher if you have any problems (that's what they're there for), even if it means staying back five minutes after class. It is better to solve little problems early onwards, than let them become cataclysmic right before an assessment or exam.