Like BIgAl says, what do you do all the time that takes more than a simple command or mouse click? Start simple and work from there - and you will learn as you go
This forum is great for advice and LISPs, often what I want has been asked before so there are ready made solutions. Another great resource is Lee Macs LISP website.
I started with 'utilities' rather than automating my work itself. For example my first one was 'Zoom All' (command line command 'ZA') which only saved typing 6 characters but.. its still quicker. The other one I use all the time is a plotting lisp, 'plota3' and off it goes, A3 plot without the dialogue box (so long as the drawing is set up OK in the first place), saves time. Worth doing I can take these anywhere with me.
After that I looked at my actual work and have a few that will automate tasks I do often. Often I will use a LISP for work more for consistency and accuracy rather than the speed boost itself (eliminating typing errors).
The 'Zoom All' LISP might be a good start, if you can type the command into the command line you can make it into a LISP really easily. Just save as a new LISP name, and change the command and away you go:
(defun c:za() ;define name of LISP, use c: to enable you to run it fromt he command line
(command "zoom" "a") ;start with 'command' to tell it your runnign a command line LISP - and then between "" everything you type in and off you go
) ;end the LISP defnition