Usually you'll try to pick two points off of the graph of the line that are clearly defined.
Having done that it's simple to compute the slope and then use point slope form to find the equation of the line.
Let's suppose that a line goes through the points (2, 3) and (4 , 6)
We can find the slope of the line as [ 6 - 3 ] / [ 4 - 2] = 3 / 2
Using this and the either of the points....we have
y = (slope) ( x - x coordinate of point) + y coordinate of point
y = (3/2) ( x - 2) + 3 distribute the 3/2 over the terms in parentheses
y = (3/2)x - 3 + 3
y = (3/2)x
And that's the equation of the line