Here is a list of basic tactics that will greatly help you in your campaign for the world (whetever each faction you choose).
Build in Mass
As with many Command & Conquer games, it is better to build one large force than a series of small ones, not only is it good for your resource count, but it will also give you a better chance to complete your mission.
Variety is the Key
In some of the Command & Conquer games, just using a huge force of one particular unit will not help you win the day, this is because every unit has a weakness, it is best to build a variety of units that can cover each others weakness, for instance, if you are going to build a tank force, it is wise to train a good number of basic infantry, this is because most tank weapons are for other vehicles, and are not always effective against infantry.
Shatter the shield
When you eventually make it to the enemy base, it is best that you deal with the defense system first, so that you can avoid heavy losses to your force, it is also good to use a unit that can attack and destroy defenses from a safe distance (I.E. Athena Cannon, V4 Launcher) and then have your forces storm the enemy base.
In some campaign missions, you are limited to the use of your basic infantry, and they can not last long under heavy fire, so train enough of these units to garrison buildings, by doing so you will increase the surviving chances of your force.
The Engineers for whatever faction, can do a lot more than just repair and capture buildings, they can capture civilian buildings (I.E. Hospitals, Garages, Oil Derricks). By doing this, you will have a better chance for completing missions as they will provide you with restoring your units health or give you additional resources for as long as they still exist and controlled by you, as the enemy will try to either capture it or destroy it.
Move out to the ocean
Command & Conquer: Red Alert 3 gives commanders the ability to build a base on water, in some situations this will have an advantage in your favor as some land units can not attack targets way out on the water, so for example, if your construction yard is under heavy attack, and there is water on the battlefield, move it out to the water. The only way it will be attacked, is by long range bombardment units or naval or air units.