Accents are not decorations. Have a reason for using them beyond "I'm writing a fantasy and they look cool." (The same goes for apostrophes.)
In addition to Daniel's very good answer:
- An accent may indicate not just spoken vs. silent but a completely
different word. See the differences between des, dés, and dès
(of, dice, and since respectively) and côté, côte, and cote here
(side, coast, and stock).
- An accent can change the pronounciation of a vowel. The umlaut (not a
native English diacritical mark) over a vowel in English generally
means that the vowel is written once but said twice: naïve is said
nye-eve, not nave. This is why you might occasionally see cöoperation, to make sure you don't think it should be coop eration.
- Accents may be required by pronounciation, or change it. The cedilla makes a C soft (François is Fran-swah, not Fran-kwah). The tilde adds a vowel to an N (jalapeño is hala-pee-nyo, not hala-pee-no).
- Sometimes accents are used to indicate historical orthography. The
circumflex over a vowel in French means that at some time in the
past, the word had an S in it. Hôtel used to be hostel, maître
was maistre, and so on.
- I don't speak or read Hebrew, but I know that it's written without
vowels, and the accent marks (dots) indicate what the vowels are.
More discussion here.
This question might also be useful:
How can I effectively invent a language?
If you want a more thorough discussion of creating a language, known as a conlang or constructed language, I highly recommend David Peterson's The Art of Language Invention. Peterson created languages for Game of Thrones and Defiance, among other properties.