There's a wide variety of tactics for playing DotA, and there's no way anyone can describe all of them (though DotA's homepage try very hard to do that, and the forum got tons of information, and many different tactics).
In this guide guide we will just scratch the surface of the tactics in DotA, with some (important) information for newbies, as well as a the average players.
Denying creeps is essential in the beginning. At level 1 in easymode, you need about 3 or 4 creeps to gain a new level. Denying an opponent a creep will give you the upperhand. You don't just reduce the amount of experience your opponents get, you also deny them the chance of recieving gold from the creeps.
To deny a creep, you must deal the killing blow to your own creeps (you can only attack your own creeps when they have lesss than 50% of their maximum hp).
Most DotA players may know this, or have experienced it, but here is some pointers about attacking towers anyway.
If your creeps are attacking the opponents creeps, and none of you creeps attack the tower, the tower will make you it's first priority if you start hitting it.
If your creeps are attacking the tower, it is safe for you to attack it without worrying about it hitting you.
An exception to the statement above is if you have lower hp than the creeps, as the towers seem to focus on units with low hp first
The tower will also start attack you, if you start hitting an enemy hero, in the vicinity of a tower. An exception to this, is if that hero already is taking damage from your teams creeps.
Always be careful around towers, it's unwise to let them knock your face in when it is not needed.
Powerups are a great boost, for several reasons. They may give you time to escape, give you a bigger chance of killing an oponent, or even if it's just giving you the chance of staying longer in combat.
Most players, if not all, know that when they are chased by enemy players, taking a detour to a spot where a powerup may be, can save their lives.
Now, people who have not played DotA at all, or just a few times might think: "500 gold for boots of speed!? Hell no! They wont help me survive or do more damage".
Well, maybe there's no player in the world who thinks so, but the point is that, without speed, you will be an easy target for other heroes, and you won't be able to kill other heroes often.
Though, to some heroes, speed is not as important as to others. This is because many heroes in the game got "blink" abilities, that teleport you a certain distance, and can indeed be used both to chase other heroes, and for the grand escape.
The lesson I wish to teach is that, always keep track of how fast your hero is, otherwise those other items you have gotten won't do you any good.