Attacking is an integral part of the game. You will find that the better player will be the one on the offensive, as opposed to the defensive. The player who is aggressive will win a game 9 times out of 10. If it is that little bit of aggression that allows a player to capture one oil derrick, then this, though a seemingly small advantage, could be enough to win them the game. A perfect example of attacking aggressively is to destroy the enemies harvesters; if an opponent has war miners that are a long distance from his base, and you throw your tanks at them - then your opponent has to decide whether or not to take his tanks out of the safety of his base, or to use them in an effort to save his war miners.
If a player successfully manages to cut off an ore field, as opposed to killing harvesters, then this could be enough to ruin an enemies economy. For example, many aggressive players will park a group of tanks on an ore field - hence cutting off the ore supply to their opponent. Many, many players will simply not combat this. They will try to think of another way to get ore. A good player will combat the aforementioned - they will do whatever is necessary to regain that ore supply - for without an income, the player is fighting a losing battle. A player has not to be scared of sending their troops out of their base into the open - as silly as it may seem, many of the poorer players are scared of leaving their base. They sit there (so to speak), as they are simply too worried of fighting a battle without their prism towers or tesla coils to back them up. Attacking is about aggression - aggressive players are, in the outstanding majority of situations, successful players. You must however balance aggression with common sense - throwing ten rhinos at twenty rhinos isn't aggressive or daring, it's plain stupidity.
Attacking involves risk. It's a matter of how good you are at calculating the risk that will decide on how good a player you are. If you can destroy an enemy tech building or a harvester then you immediately have an advantage - but you must make sure that your opponent doesn't respond. You have to study your opponent, you have to know your opponent and you have to try to read your opponents mind. If you can guess what your opponent is going to do next, then you can easily combat it. A good player always studies his opponents moves - and is always one step ahead. Strategy makes heavy use of the brain. You need a fast thinking, decisive and tactically aware one to function as a good player.
Another rule to follow is to 'expect the unexpected'. A good opponent will, in many cases, do something totally against the run of the game. For example, you may both be throwing groups of rocketeers at each other, but then your opponent may build a nighthawk, fill it with engineers, and land it in the middle of your base and take all your structures over. Tip - If you ever hear a nighthawk then it means your opponent is most likely going to fill it with engineers and try to sneak it into your base. In most cases they will send the nighthawk around the edge of the map, to prevent you from seeing it. If you manage to destroy it with a group of engineers inside it, then your opponent has lost $3000, a lot of money.
To be a good player you must always attack. You must put your opponent on the back foot from the offset, and then make sure he stays on that back foot. Defending is very rarely the method used to win by a good player, very rarely indeed. If being aggressive and attacking means destroying one harvester or one tech building, then it could be enough to win you the game. Never be afraid to use your units in a win or lose attack - risk is a key factor of any good players game. All tactics involve risk - however it is often the more risky tactics that are used by a good player. War is about risk, war is about taking a chance - it is how well you can judge the risk factor involved with a manoeuvre, that will decide between the better player, and the average player.