A bit of practise is enough to beat the easy computer, then a tiny bit more for the hard one, but if you want to try playing multiplayer games, you pretty much have to see replays. I suggest
www.wcreplays.com. Download some replays involving the race you want to play, watch, see what's making them so good and try to include it in your own playing.
To watch replays, you need to have the latest version of the game (acquired by connecting on BNet), and to put the .w3g file in the warcraft III/replays directory.
The units you use in a specific game is up to you, the understanding of the complex rock-paper-scissor game the unit choice is come with time (and the knowledge of armor and attack types), but something really important in Warcraft 3 is the BO, or build order, which is basically what you do in the first minutes. More experienced, you will want to adapt it for you own tactic but starting by copying some skilled player one is enough. Try, as they do, not to lose time on this part of the game. It's actually very easy and gives you lots of precious seconds of creeping (killing neutral monsters for XP), rushing (attacking as fast as possible with few units), harassing (trying to kill workers, steal loots, steal kills or kill damaged units without engaging fight), or whatever you want to do.
Some to-do things :
Use the alt key. It's important.
Focus on one enemy unit at a time usually, unless it prevents your own units to attack. If one of his units is already damaged, finish it.
When one of your units is going to die, you should make it flee battle. Then you can either heal it later or make it return in the fight when it can attack without retaliation. If the enemy pursue your fleeing unit it can be good because, well, he won't be attacking you while doing so.
Try to kill the most annoying units, being usually the most damaging or the heroes, in priority, but don't lose too many troops doing so.
GL & HF, as they say.
[ January 10, 2008: Message edited by: Cyx ]