2D Interpolation

I''m trying to interpolate between two 2D vertices by using a start vertex, a finsh vertex, and a 0.0 - 1.0 float showing how far it should be interpolated. Now, the problem is that I don''t have the foggiest idea on how to implement it correctly. I''ve messed around with a few different "techniques", but I can''t get it to work I dunno if I''ve explained that very well, but any help would be much appreciated. --------------- #define TRUE 0 #define FALSE 1 //MUAHAHAHAHAAAA!