the best way to start is to take a clinic with a friend..find a local pro and try it out... if you go out on your own and are not very good, it will become very frustrating for you and youll end up quitting..Find a friend at your same level and a teaching pro that will make it fun to learn!! also, look into cardio tennis classes, and any beginner social leagues or round robins in your area..good luck!
i currently teach tennis part time. a lot of my students found me on craigslist. i would go to the local parks and rec. dept. in your area and see if there are tennis classes available for beginners. since you are just starting, i highly advise you to stay away from regulation balls because they will be too fast for you. i would go with the foam balls or even the low compression balls where the balls bounce much slower. ask if the instructor uses them. i swear up and down by them. my wife who played for only the second time had a blast with them as well as my 9 year old. anyways, i hope this helps, i wish you the best.