Spelling in English can be hard. A misspelled word often ruins the impression of your writing. Wrong spelling in your essay will seriously reduce your overall score. At times it can be as simple as just paying attention to what you are writing. Other times though it can be tricky, especially when it comes to words borrowed from other languages, namely the French ones (like connoisseur).
A comforting fact is that ability to spell has little to do with your overall intelligence. Many famous writers were known for being very bad at that (spelling, that is).
How to improve your spelling
The one and only surefire way to increase your spelling literacy is to memorise words. Reading helps and so does looking words up occasionally whenever you have doubts about the correct way of spelling them. You can use flashcards to revise problematic words. Anki is probably the best free software that help memorising things — vocabulary, spelling and much more!
As you should already know by now, there are minor differences in spelling and more serious differences in vocabulary. Even though mixing British and American vocabulary isn’t punitive, you absolutely have to be consistent with your spelling. So if you choose to stick to British spelling (e.g. centre, cheque, judgement) then omitting “u” in “valour” or “labour” will be regarded as mistake and scored accordingly. Here is a basic reference table to AmE/BrE spelling:
Color, valor, honor, humor etc.
Colour, valour, honour, humour etc.
Theater, center, fiber, liter etc.
Theatre, centre, fibre, litre etc.
Realize, analyze, apologize, colonize etc.
Realise, analyse, apologise, colonise etc.
Traveler, enrollment, fulfill, jewelry, marvelous etc.
Traveller, enrolment, fulfil, jewellery, marvellous etc.
Offense, license, pretense etc.
Offence, licence, pretence etc
Words with difficult spelling