lerp(a,b,x)
a - Starting number
b - Ending number
x - The percentage amount between the start and end number you want to get (between 0 and 1, or 0% and 100%)
So lerp(10,20,0.5) gives you 15, or halfway between 10 and 20.
Allan gave a pretty good explanation of why it slows down above - basically when you use the current position as a start point, the lerp amount gets updated every tick to be smaller and smaller.
Just going to give you a heads up though - lerp is generally used for more advanced gradual motion/inertia. If you're looking for simple straightforward movement, you would best be served by getting familiar with the Bullet behavior. The Bullet behavior is generally the go to solution for almost all linear movement needs.
Alternatively, http://c2rexplugins.weebly.com/rex_moveto.html is a fantastic plugin that you might find useful.