Accent marks above letters?
I don't quite understand how to know when the accent marks are to be used. is it just a memorization thing or is there some easy way to know when and on what letters they go above?
Yes, it is a memorization thing. As for any languages, two words will have different meanings by just placing accent marks on them. For example qué means what, and que means that.
This artircle may help you accent -SpanishDict
In some cases, it is a memorization thing, as aurelioque20 said, like in the case of some one syllable words and question words. They have accent marks when asking a question but don't when they are answering a question. For example, dónde would be used when you want to ask where something is, but donde is used to tell where something is. Like this: Where (dónde) are my keys? They're where (donde) you left them. The same goes for que/qué, como/cómo, etc. More often, though, a simple set of rules apply. If the word ends in a vowel, an n, or an s, the emphasis is pronounced on the second to last syllable. If the emphasis is somewhere else, you have to put an accent on that syllable to indicate that it should be emphasized. If the word ends in any other letter (a consonant that isn't n or s), the emphasis is on the last syllable. If it's pronounced somewhere else, put an accent on the syllable that should be emphasized. As Moe and Fidalgo mention above, there are several helpful reference articles on such things here on SpanishDict.com.
Those little "accents" or "tildes" (written accent marks) let the reader know where to place the emphasis (or stress) when a pronunciation rule is altered. Understanding them is not entirely memory work.
We have two Reference Pages that will be a big help to you in understanding where the stress or emphasis is placed on spoken Spanish words. One page explains the standard or usual rules, and the other deals with exceptions to the usual or customary practices. I suggest you will find it helpful to read them in the following order:
This one explains usual rules for stress ----> Emphasis
This one explains the rules for the use of accents ----> Accents
When you have read the pages and have understood them (they are not difficult) you will have the better understanding you are looking for.
Muchos salodos/Best regards.