Question

how do you read a coordinate grid? How do you know which one is the y and which one is the x?

Answer 1

It is just a convention - there is no reason for it - BUT

the X is always the horizontal axis, and the Y is the vertical

when you right a coordinate the X is always given first

Answer 2

x-coordinate is a horizontal shift (how many units point going left or right)
y-coordinate is a vertical shift( how many units its going up or down)
for signs