You know which is the x coordinate and which is the y coordinate because (x,y) is an ordered pair. The x is always first and the y second. For example, the point (4,5) has x coordinate of 4 and y of 5.
Zero is always plotted on the Y and X axis, known as the origin.
The Y axis is vertical, like a straight line or pole for better reference, the X axis is horizontal, like the horizon.
It could be a velocity graph or an acceleration graph.If the plot is a straight line it is constant velocity.If the plot is a curve it is acceleration.