There are broadly speaking two ways to be good at spelling.
The first is understanding spelling. For a basic understanding of spelling, you need a quite thorough knowledge of the way words are formed from their constituent letters and how various combinations of letters typically stand for certain sounds (this method is called phonics, and is what they use in primary schools in the UK these days). For a more sophisticated understanding, you really need familiarity with a couple of other European languages and an appreciation of the history of their influence on English. All that takes years to master – so much for understanding!
The second is remembering spelling. A lot of this is done without conscious effort, simply through your familiarity with the words through regular reading and writing. But sometimes those spellings just don’t stick in the cerebral cortex, so you have to give your memory a helping hand. There is learning by rote, for example, simply repeating aloud or writing the spelling again and again and again. This can also help you get to sleep. Another tack is to find give yourself something more stimulating to remember than an arrangement of letters – a silly rhyme, a nonsense story, a pun, an acrostic poem, anything like that. One coffee, two sugars for ‘necessary’ is one that has always stuck in my head. And I call the change from ‘y’ to ‘i’, in beauty and beautiful and similar words the ‘Why Aye’* rule. Google will help you find one for almost any spelling, but you might remember it just as well if you make something up yourself.
Anyway, I took the trouble to find some resources to help you tackle the errors that can lose you vital marks come assessment/exam time.
The first is the OUP’s list of common spelling mistakes, with a handy printable exercise.
The second is an American website with 100 common spellings, and a way to remember the correct ones.
The third list will be especially useful to those for whom English is a second language.
Find at least five words on the list that you may have misspelt in the past and are likely to use in your exam or an assessment. First, think of a way to remember the spelling. Then for each word write a sentence that you might use the word in.
(* Note for non-British readers: ‘Why Aye’ is an example of the Geordie dialect, spoken on and around Tyneside. It means ‘of course’. Knowing that may or may not make the joke funnier.)