I remember very well the real struggle of trying to figure out what word should go in that empty spot. The best tactics I have for you to get better at word-formation are: solving past papers and reading in English. Many examples from the previous exams will appear on your test, so it is worth solving these. Reading will expose you to a wide range of vocabulary and new words. You will start to intuitively form correct words because you have seen them before (even though you may don’t remember where). There are some rules for word formation, but studying them may not be as effective as using these two tactics.
Here you will find great resource that will give you a clear understanding of suffixes and prefixes (and it will not kill you with lengthy rules):
https://dictionary.cambridge.org/grammar/british-grammar/suffixes
https://dictionary.cambridge.org/grammar/british-grammar/prefixes
Good luck!