How To Stop a Tank Rush
One of the first things people come across when they are starting to play the game is learning to stop a rush. How do you do it? What should you do? Well, the first step is knowing it is going to come before it even starts: anticipation. The next steps are being able to stop the rush with defensive buildings and fodder. This is a very basic guide and is meant to be aimed for beginning players.
Obviously if you know it's coming you can start up your defenses or think of a strategy to stop it before it even starts. This might seem obvious, but if you've played the player in the past and you know they like to rush, you should clearly be watching out for it. This is going to happen in scenarios where the opposing player thinks rushing is going to be good. Situations that this is true in general are when you are the allied player and they are soviet in smaller to medium sized maps. Also, in smaller maps in general. Basically, you want to know it is coming before he is already in your base. A really easy way to combat this is by scouting well early.
Scouting is very critical if you are going to be able to stop an attack and it is going to be useful to see the entire terrain of the map anyway, Also, in situations where you basically KNOW the other player is going to be attacking early, building extra dogs early on when you are going to have to get fodder (dogs) early on anyway is a good idea. By getting extra dogs you can scout the map more effectively, anti-scout, and then have fodder for later. This is an especially good idea on a map like Tournament Map B as allied vs. a soviet player when the soviet player is almost always going to put the pressure on early. Make sure when you are done scouting you bring your dogs back to your base! It's easier to do this if you number your dogs early on so you can quickly select them and bring them back later. You can do this by selecting units and hitting ctrl+1, ctrl+2. Later on when you send them back you can select one dog and hit "T" to select them all. Also, it is critical that you always watch their war factory at the beginning of a game to see if they get a miner or tanks first. It's going to be a bad idea to go more than 1 miner more than your opponent. For instance, if you originally planned on going 2 miners on a map, you should first just click to build one, watch your opponent's war factory and, under the assumption you are both building the same speed if you see nothing come out of their war factory when your miner is 3/4 done then you can start the next miner. Just be sure to be ready to cancel it quickly if you see a tank come out! Next up is being speedy!
The slower you are at building up your base the bigger the difference there is going to be between how much your opponent has and how much you have which, again, obviously puts you at a disadvantage. Specifically, this difference is going to be a bigger deal on smaller maps. So, commander! Build quickly! You can build faster by knowing ahead of time how you are going to build. If you have a build order (BO) already planned ahead of time you won't waste as much time thinking about where to place your buildings. Also, you can place your buildings quickly by using the default hotkey "q". Q is a hotkey that will automatically select ready buildings under the "Home" building tab. This means you don't have to click the ready building and then click again on the terrain to place it. This saves you critical time. When a building is close to done place your mouse arrow over the area you wish to place it. When it is ready, quickly hit q to select it and click to place it down. Now it is time to defend that base of yours, commander.
3. Defense: Home advantage
Great, now we know it is coming and we are building equally fast with our opponent, but what is next? Well, that is where we start with defense. First and foremost, you have the home advantage. This means you can place sentry guns/pill boxes to help defend your base. DO NOT GIVE UP THIS ADVANTAGE. I see many players move outside of their base when trying to defend just that--their base. You have an advantage available to you as your tanks are coming out right there and you can be placing defensive buildings. By moving further away from your base you are giving up this advantage. Do not move away from your base.
3.1 Defense: Pill boxes and Sentry Guns
The basic defensive building is the pill box and sentry gun. These are vital in stopping a rush. They are effective vs. infantry and can do decent damage vs. a small group of tanks (3-4). They also act as fodder for tanks--taking hits that your tanks would otherwise get, and are going to be the biggest key in stopping a rush. How to use them effectively depends on a number of variables that are all dependent on many things, but the main idea is to deploy them in key locations and at key moments. Learning where and when to place them is going to take a lot of practice, but in general you do NOT want to place them ahead of time. When players start out they often place these defensive buildings early on, but this is actually a mistake. It is easy for the aggressor to simply circumvent these defensive structures rendering them all but useless and a waste of money. Instead, you should be placing them right as the enemy is coming at you and in front of your tanks and fodder. This may be easier using the defense tab hot key "W" which works the same way as the "Q" key introduced previously. If you do have fodder then as you place the sentry/pill then you should be moving toward the enemy as he will likely move away allowing you to get one or two free shots in as he retreats. But remember, stay in your base! Don't go to far out.
3.2 Defense: Pill boxes and Sentry Guns as a blocker
In the X diagonal directions a pill box or sentry gun can more effectively be used if your tank is immediately behind the pill or sent and the enemy's tank is firing at it. What I mean by this is that if the set up is something like this:
T = your tank
O = pill or sentry
E = enemy tank
The enemy tank will be trying to fire at your tank, but will be unable to. The pill/sent effectively blocks the fire of the enemy tank and takes the hit instead. This is critical in smaller tank battles when the enemy only has 2-3 tanks in your base. It's important to realize, though, that you can only utilize this in certain directions. Getting sentry guns and pill boxes early on is going to weaken your economy, so that is what we should tackle next.
Depending on the map and situation, it may be best to stop production of buildings and focus on getting sents/pills and additional fodder so you don't stop building tanks. This is going to be more likely on lower economy maps with no derricks or gems. Also, if you got an additional miner than your opponent in the beginning which is likely to be the case, it may be that your opponent sold his MCV for additional funds and fodder. If this is the case he is basically going "all-in" and risking everything to win quickly. If you have the additional miner and are very low on funds, this is the time to sell your mcv and get more fodder, just be sure you are making the correct tactical choice here because once you sell it you will not be able to build pills or sents.
You can find a more advanced guide for economy by me found here: http://xwis.net/forums/index.php/topic/177567-sekes-guide-to-miner-control-and-refinery-placement/