The y-intercept is where the graph crosses the y axis. So if you are given an equation in slope-intercept form (y=mx+b), plot a point at y=b.
For the slope, think rise over run. For example, if you are given the slope m = -1/2, you should plot a point 1 below and 2 to the right of the y intercept.
Then just connect these two points with a straight line.