One thing I notice about newbies is that many of them play humans. I think that this is because many of them haven't gotten far enough in the campaign to see any magic, and therefore see no difference between the two races, which we know is complete bull. This is not good for them. Why? Humans are much harder to use than orcs, and I'll explain why below. While this doesn't make them worse than orcs, it requires the player to be extremely skilled at Warcraft, which most newbies are not. So you should only play humans if you're looking for a greater challenge. I should also add that you generally get mroe respect when you win as humans than when you do as orcs, so if you're going for the glory, play humans.
Human Mages are arguably the most powerful unit in the game when used correctly. You can decimate legions of Orcish (or even enemy Human) troops with a single mage if you know what you are doing. Fully upgraded Elven Rangers are also much more powerful than their Troll counterparts (+3 damage is better than regeneration, and you can heal your guys anyway). Human Paladins also have Exorcism, which allows them to kill Death Knights and Skeletons from a safe distance. Even Healing can also give you the edge in a close battle.
The biggest weakness of humans is that they don't have bloodlust. This Orc spell triples the damage that the unit it is cast on can do, and it is a major problem for human players. There are many ways to stop bloodlusted ogres but none of them are as easy as just bloodlusting your troops. Humans must be extremely aware of their surroundings, having a couple of flying machines around your base at all times is essential. The ratio of Paladins to Rangers to Mages is also very rigid:too many paladins, and you won't be able to kill ogres. Not enough paladins, and you won't be able to kill ogres. On a lesser note, Exorcism is completely useless against other Human players.
First of all, before you even build your town hall, you should right click (or command click for mac users) on some nearby wood. If there isn't any nearby wood, click on where you think wood is on the mini map and see if your peasant's Harvest button turns green. If it does, then you build a town hall. Don't spend more than two or three seconds on this though, or you may not recover from the speed loss. This will give you an extra 100 lumber on completion of the hall, which can help significantly in low games, and to a lesser extent in medium and high games. It works for lumber mills too, and in the early game that extra bit of lumber can be all the difference in the world.
Assuming you now have a town hall, the first thing your peasant should do now is build a farm. This peasant should continue to build farms indefinitely, possibly until the end of the game. As soon as the farm is done, train a peasant in your town hall. Keep training peasants back-to-back. Your first peasant should build your barracks. Keep in mind that units come out of the top left corner of a building when they're created, so you want to keep clear passage from that area. Otherwise, your units (not to mention your poor peasant) will get stuck, and you will have wasted gold and time on them. You can chop them out, true, but it's better to keep them out in the first place. Also make sure your guys don't have to travel really far to get to the entrance, unless you want to manually order your guys to move to defend from an attack. Your third peasant does the lumber bug (see above) and builds a lumber mill. The fourth peasant builds a blacksmith. The next ten peasants go on gold (you will be in desperate need by now) and after that, every third peasant chops wood. The guys who built the barracks, lumber mill, and blacksmith also go on wood. Once you have enough gold to train footmen all the time, build a second barracks. By now your guys should be about level 3. When the barracks is done, your next 3 peasants go on wood, the fourth joins the first building farms, and then back to 2 gold, 1 lumber. When you have two barracks, 10 or 11 footmen, 20 or so peasants, and a big cash flow, go for keep.
As soon as your barracks is done, begin training footmen constantly. But don't give them the highest priority. If you have enough gold for a peasant or a footman, get the peasant. Enough for an upgrade or a footman, get the upgrade. If you have enough for a farm or a footman, check your food. If you have 6 or 7 available go for the footman, but if you only have 3 or 4 get the farm. I notice that many people never make any footmen at all, they just go straight for knights. This would be a sound strategy if nobody attacked in the early game. The problem is that you will almost always get attacked in the early game. Get as many as you can, as fast as you can. If you're worried about them being a waste later in the game, see below for tips on what to do with them. Like it says above, when yo have 10 or 11 footmen go for keep.
During the keep upgrade the action reaches a lull point. The hectic pace of the early game has ended, but the hectic pace of the mid and late game has not yet begun. This time is not as crucial as the early game, but is still important. As soon as you hit that Upgrade to Keep button, you should click that last weapon upgrade at the smith, build another barracks, and start training archers at both barracks. When the third one is done, train archers there too. During this time continue making farms and such. You may want to send an archer out to scout the territory around you and find your opponent. As the upgrade nears the end remove a peasant from your gold line and have him stand apart from the rest. When the keep upgrade is done, the real action starts.
As soon as the keep gets finished, you should do three things. Go to the lumber mill and upgrade to Rangers. Go to the Keep and train peasants. And remember that little peasant who got moved up there? He builds your stables. When the Ranger upgrade is done, the rest of the upgrades should go as follows:Marksmanship, Longbow, Scouting, Weapon 1, Weapon 2. Rangers are a great help when fully upgraded because they can do so much damage. As soon as your stables is done, stop making rangers and make knights. You should also make another barracks. When that is done, alternate between making 2 knights, 2 archers and 3 knights, 1 archer. This will properly balance your army so you can deal with early bloodlust. When your fourth barracks is done and you have plenty of gold, go for castle. You should not have a keep very long, only long enough to make your stables and get a few knights.
During the castle upgrade, you should be pumping out knights and rangers in the pattern above. After you hit that button, start a Gnomish Inventor. This will produce about 6 or 7 fliers before you start on demo squads. Now is also a good time to start a fifth barracks. When that's done, your army pattern should become a constant 2 Rangers, 3 Knights. By now you have probably been attacked. If not, well, that's beter for you. Be ready for more fast paced building when the castle is done.
If you manage to make it to castle, and get mages before you are killed, the game is almost certainly yours. But to guarantee that, here's what to do after the castle is done. When the castle gets finished, immediately start two mage towers and repair them. As soon as they are done, train a mage in one and blizzard in the other, and start two more. When they are done, get slow and invisiblity. When the first mage and blizzard are done, keep training mages in those towers while you get the last two spells (Polymorph and Flame shield) when Slow and Invisibiltiy are finished. After that, make a LOT of mages. In an army of 150, about 60 should be mages. When your mages are at level 4 or so, make a church and get paladins. If your opponent is orcs, get exorcism first, and if he's human get healing and don't ever bother with exorcism. When you have level 6 mages and level 8 paladins, you can begin attacks if you want, or make more troops. By now your mine is probably pretty low, so you may want to expand. It's your call from here on. But you may want to use some tactics on the Offense Page to soften them up first and make the battle easier.