In case you are asking how to use this snippet of code, copy and paste it into Notepad, save it as PTT.lsp somewhere that AutoCAD knows (in it's search path), then when in Autocad, load it using a pull-down or command-line (Load"PTT"). Then type PTT and go. You could also copy and paste it into your Acad.lsp file so it would always be available. If you are using any version of LT, forget it all.
If you are asking how to learn LISP, my advice is to read other people's code and experiment. Autocad Help will list the commands available. There are books out there, lots of them. There are very helpful web sites also - just search for LISP. If you have any experience programming in almost any language, you can pick up LISP in a heartbeat.
Each word in LSIP means something easy. For example (setq variable expression) means evaluate the expression and place the result in variable A. It's like learning French. Master the alphabet (ie LISP commands), get to know the grammar and then learn from the work of others.