If you are only missing b(the y-intercept) but are given a set of points, plug the points into x and y and solve for b.
If you are only missing the slope(m) but are given a set of points, plug the points into x and y and solve for m.
If you are given the standard form/point-slope form, change the equation to slope intercept form.
If you are given an complete form(there is an x and y; no missing variables), but are not sure what it is, plug in some numbers in x to find y, then graph.