Draw the students a straight line and explain that it has a property called its slope, and that the slope is represented by the letter m.
Show the students where the intersect point is with the y-axis, and explain that the letter b is used to represent the y-intersect.
Introduce the slope-intercept formula: y = mx + b. Use simple examples, and have the students draw lines corresponding to your examples. Always emphasize that there are only two things to remember so far: m and b.
Have the students form pairs and have them take turns giving examples and drawing lines while you monitor their progress.
Give the students the two-point formula y - y1 = m(x - x1) and stress the fact that y1 and x1 are simply the coordinates at one point on the line. Do this without showing the students the general origin of the formula.