How to increase range of AM radio waves?
As others have pointed out, you first need to ensure that you adhere to the relevant standards for TX power in your area, and use a legal frequency.

There are things you can do to get better range. One of them is to make a more sensitive and selective receiver. You could experiment with making a tuned loop antennae - basically a large coil with a cap, forming a tuned circuit, tuned to your transmission frequency. You use a single turn as a feed to your receiver. They are quite directional, so you want to mount it so that you can tweak the position. I've had good results with one of these for picking up UK Am broadcasts on 198kHz, from about 500km away. This should certainly increase the range of your transmission.