We learn the most about Nick Carraway, the narrator of The Great Gatsby by F. Scott Fitzgerald, in the opening pages of the novel. He tells us where he came from (the Midwest) and what he has been doing since college. He also tells us where he is living and why he is here. He also tells us about the advice his father once gave him. Nick says this about his father:
"Whenever you feel like criticizing any one," he told me, "just remember that all the people in this world haven't had the advantages that you've had."
Because of that, Nick is certain that he has a gift few other people have. He says,
[i]n consequence, I'm inclined to reserve all judgments, a habit that has opened up many curious natures to me and also made me the victim of not a few veteran bores.... Reserving judgments is a matter of infinite hope. I am still a little afraid of missing something if I forget that, as my father snobbishly suggested, and I snobbishly repeat, a sense of the fundamental decencies is parcelled out unequally at birth.
This concept of withholding judgments is significant for a storyteller because, if indeed it is true, the listeners (or readers) can trust whatever the narrator says. If he is not a reliable narrator, we (readers) are going to have to work hard to determine the truth of everything Nick says. Unfortunately, since Nick is the person telling us about himself, we have to wait until something actually happens in the novel to decide for ourselves whether Nick truly does "reserve all judgments," as he claims he does.
I would like to give you some examples from the story to examine, but by your question I presume that you are just beginning the novel and I do not want to ruin the story for you. Instead, here are some general guidelines for testing a narrator's reliability. First, examine what other characters say and do and then "listen" to how the narrator describes those same things. Second, determine what other people think and feel about the characters the narrator talks about to see if there is consistency. Finally, listen carefully to what else the narrator says about himself and see if he has a realistic and accurate view of himself based on what you see and hear.