As Shaktal points out "The general equation for a straight line in Cartesian co-ordinates is y=mx+c, where m is the gradient of the straight line and c is the y-intercept of the line."
So you need to find the gradient or slope 'm' and the y-interecpt 'c', in other words the value of y where x=0.
To find the gradient, since the equation is linear, you can take any two points (a, b) and (c, d) and calculate
Then once you have that number m, you can take any point (x, y) and write y=mx+c. Then you can figure out the value of c such that this equation holds true.
As an example say you have (1, 4) and (2, 10) as the points for which you want to find a linear equation. m=(6/1)=6. Then considering (2, 10) we can write 10=(6)(2)+c or 10=12+c. So, c=-2, and our linear equation reads y=6x-2.
If you hand-plot the points, draw out your co-ordinate system, plot the points, and then connect them with a ruler or straight edge.