First, consider the 3 points, no 3 of which are collinear; as shown in the diagram below
As one can see, we can form 3 different lines, lines 12, 13, and 23 (this is 2+1=3).
Similarly, in the case of 4 points,
There are 6 possible lines when considering 4 points on the plane (3+2+1=6).
Finally, in the case of 5 points on the plane,
4+3+2+1=10 lines when 5 points.
Therefore, for 18 points, there are 17+16+15+14+...+3+2+1=153