When I first started learning tennis, I thought it should be pretty easy to pick up.  I am pretty competitive and have been playing all types of sports all my life.  Boy, was I wrong!  Tennis is a totally different beast.  I watch how the pros play and notice that most of the players in the public courts just suck.  Not wanting to settle for less, I began my quest to find better instruction trying to get better in the game.  I can tell you all the instructions on site like, D106, eHow, expertvillage (the worst) are just plain wrong.  Or god, you don't want to play like them.  Most of these armchair coaches use a racket bigger than their body, if I want to play like that, I don't need anybody to teach me, I just need to break my arm.  YouTube is a better source of tennis instruction, since there are seemingly unlimited supply of pro practice videos and well, more armchair coaches.  The problem with YouTube is that most videos are in 30 fps.  Meaning that it's real time and you will miss out on all the important info.  If you slow down a 30 fps video and go frame by frame, you will just see blur.  If you are learning from the pros, make sure you search for super slow motion videos around 240 fps.  Then you can watch more comfortably and do real frame by frame. 

Some of the better site I came around are KickServeTennis.com and TennisMindGame.com.  KickServeTennis.com is more for 3.0 or above while TennisMindGame.com is for beginner to 3.0 ish.  If you are new to tennis or sports in general, TennisMindGame is a good place to start.  If you are generally good in other sports and want to make the jump from 3.0 to higher, check out KickServeTennis.com.

Remember, tennis is supported to be fun and easy.  If you found it hard to learn, you should probably switch your coach.  Good coach is hard to come by, since they are paid by the hour, they don't really have incentive to help you get better, they want you to suck and come back for more.  That's why tennis coach usually either talk for half a class (group class) or just keep feeding you ball and "describe" to you what you should do.  Why don't you just show me?!?!?  Because they are not that good either!!!  You are much better off learning from the best:  Rafael Nadal | Attacking Forehand