well you have to know how to do algebra, and it uses the equation Y=MX+B
M= the slope of the line B=were the line crosses the y-axis
so lets say
Y=1/2X+2 Would mean you make a point on (0,2) because B=2, then after you make the point the fraction 1/2 would mean up 1 over 2, and so fourth tell you run out of room, then you do the opisite to go down and down 1 over 2, until you run out of room, you connect the dots, and you have a line that if you picked any point on the line and replaced Y and the X on the equation, it would make the equation equal... thats algebra for you :)