It's a tough game, tennis.  Not only technique, co-ordination, reaction time, speed about the court and mental resilience (and, of course, practice, practice, practice just like anything you want to be good at), but there are the rules of the game too. On top of the rules, just like any other sport, there's etiquette - things you should and shouldn't do that aren't stipulated in the rule book.  Here's a link to essential etiquette knowledge, courtesy of Local Tennis Leagues.

To help with the basics, we've put together a mix of things (rules and etiquette) that people may not be aware of, but that are important to appreciate. Awareness of rules and etiquette in general will help you to stand out as a considerate player, regardless of of your standard.

• When others are playing a rally do not walk across the court behind them, and don’t open the gates behind the courts they are playing on until their rally is finished.
• If a ball from your court rolls on to or behind another court, wait until the players on that court have finished their rally before you retrieve it.
• If it is possible to stop a ball from your court rolling on to another court while a rally is in progress on the other court, make every attempt to avoid interrupting their play.
• If a ball comes on to your court or behind your court from another court, return the ball to the original court as soon as is practical, but bear in mind the above!
• Be considerate of other players on other courts in terms of how many balls end up on your court - if you have more than 4, then another court has less then 4 so keep an eye out and, when the opportunity arises, pass any extra across to another court that has less than yours.
• If possible, leave bags, water bottles, etc at the side of the court you're playing on (by the net), rather than at the back.  
• You're not allowed to lean over the net to hit a volley - you can strike the ball on your side and allow your racket to cross the net after the strike, but if you strike the ball on your opponent’s side of the net you automatically lose the point. There is one very rare exception to this where, if the ball crosses the net with very heavy backspin, then kicks back across the net without being touched, you can then strike the ball on the other side of the net, but this is a pretty rare occurence!
• If, during the course of a rally, you strike the net with your racket or any part of your body, you automatically lose the point.
• When serving, you lose the point if either of your feet is on the baseline line or inside it at the point you strike the ball. This is a foot fault and if you are foot faulting it is not unreasonable for your opponents to ask you if you're aware you may be footfaulting.