So you've got to where you can reliably beat the AI on certain maps, and you think you can now beat humans. Well, you can't. The biggest difference between a computer and human opponent is that you can no longer pause the game to rest and think. This means that you must commit an awful lot of information to memory, and because the action happens so fast, it needs to be reflex memory.
The following points are generally true for any RTS game, although Warcraft 3 has some important unique features. You will need to put in a lot of effort to get better at this game. Before you can even think about taking on a human opponent, you need to meet certain prerequisites. First of all, you need good hardware, such as a very responsive mouse.
Second, you need to know all the game functions, such as the use of the Town Portal scroll.
Third, you need to pick one race, and play only that race until you are good with it. Not only are there four races, which is quite a lot, but they all need to be played differently. You need to memorize all the keyboard shortcuts for your race, and always use them. You need to know at least one good build sequence, and use it diligently. You need to know several good force compositions, and restrict yourself to those. Bear in mind that certain force compositions are good against certain enemy forces, but probably weak against other forces, so you will need to switch around. Of course, random collections of units are almost certain to lose all fights.
Fourth, you need to limit yourself to playing on a small number of maps, and memorize all the key features of those maps. Not only will you need to know where the expansion locations, base locations, choke points, and so forth are, but you will also need to know where all the creep groups are, and how strong they are.
Fifth, you need to learn how to scout, and you need to invest a lot of energy here. The AI does not scout, because it always has full map knowledge. Playing against the AI, you can get away without scouting, because it always follows the same algorithm. If you don't scout against human opponents, you will lose consistently.
Sixth, you need to become good at the macro game. Keep a steady supply of gold coming in, lumber as well, of course, but that takes care of itself after a while. Stay in the no-upkeep regime as long as you reasonably can, and then in the low-upkeep regime as long as you reasonably can. However, you cannot simply build static defenses ("turtle") because then you concede map control to your opponent, who will then take all the resources on the map. It is possible to turtle against the AI, because it will keep attacking you with insufficient force.
Seventh, you need to become good at the micro game, and this is critical in Warcraft 3. Proper use of control groups is essential here.
Eighth, you need to develop your strategic sense. In order to succeed at this game, You've got to know when to hold, know when to fold, Know when to walk away, know when to run; Don't you ever count your money while you're sitting at the table; There'll be time enough to count it when the dealin's done.
Finally, there are many good advanced and expert guides available on the internet. Seek them out and study them. This will save you a lot of time, although you still need to put in a huge amount of time to become good. Even then, you will never be as good as the experts, because they do nothing else but play the game.