Hi sky. The usage of apostrophes is something that even native speakers get wrong all the time, myself included, so don't feel bad. A basic rule you can try to remember is for singular nouns you add an apostrophe + an s (this isn't always correct but for the majority it will be) and plural nouns just add an apostrophe at the end. Easier examples of using apostrophes is when two words are combined into one "I'm, didn't, can't, you're" just to name a few. Look up some nouns and plural nouns and practice a bit and you'll start understanding it a bit more. Cheers :)

Here, “my exam marks” is best. If you have the well-known book English Grammar in Use by Raymond Murphy, consult Units 80 and 81. As a brief summary: Noun+noun is preferred when not referring to “ownership.” Exam marks City library Noun’s + noun is preferred when referring to ownership. My father’s car The city’s library (but city library is better) And noiun of noun for other cases. The library of the city