First of all the most important thing in just about any multiplayer and / or tactical game is knowing the lay of the land. Take time to scope out your surroundings. If you need to start over, that's fine, but understanding where tiberium is located can make a huge difference in a match. This is of course advised in a single player skirmish map. When planning out the start of your match it is good to find the best available position for acquiring tiberium. More tiberium means more resources for constructing buildings and training units and this is of course a very good thing. Running out of tiberium at an inopportune moment will almost always end in a loss so do your best to make sure that you have a continues, steady flow of tiberium coming in and survival will remain a possibility. If you are starting a match on a map with patches of tiberium that are close together, as long as it is not very far away, don't be afraid to pack up base and move to a better vantage point, this is usually best at the beginning of the map. Once you have reached the set cap of tiberium you will be told that you need to build silo's. This is not recommended, odds are if you are hearing this a lot than you are doing something wrong. The idea is that you want to be spending tiberium as fast as possible, and as long as you are spending you shouldn't be reaching the cap. Just as it is important to know the map you are fighting on, it is also important to be familiar with the faction you are using. Knowing where everything is and what you need to expand your army is key, the less time it takes for you to figure out how you want to further expand your army the better. If tiberium is getting tight or the enemy is attacking a base that you no longer need. Don't be afraid to sell unwanted buildings for tiberium, doing this has saved me many times.
C&C 3 is a very fast paced RTS. In other games such as Starcraft or Warcraft, there is a certain amount of leisure that allows you to think about your next move, plan out your units and so forth. If you take this sort of laid back approach in C&C you will lose. You must constantly be on your toes and ready to defend as well as attack. If you often find yourself waiting for a building to be constructed or units to be trained, you are setting yourself up for failure. To ensure victory you must make sure that you are never at a standstill. When something is in que, quickly move to the next thing you need, make sure there is always a steady flow of production, because while you are sitting there idle, your enemy is not. Could you use more power plants? How about turrets? Another War Factory is never a bad thing! It is just as important to make sure that you are spending tiberium as it is to make sure you are earning it.
In some RTS games it's a good idea to build up your defenses before preparing to strike. Though some turrets can help your defenses greatly, in the case of C&C, your offense truly does make the best defense. While you amass your army, strategically place your units so that they are properly defending your base. Mixed unit tactics is a very good idea here, but with the GDI, I tend lean tword ground units. With air units you need to build extra buildings and add some upgrades and though this is not an enormous hassle, that time is better spent on increasing your army, as there is little your air units can do that a massive mob of ground vehicles cannot. Once you have gotten close to reaching the cap for your units, this is a good time to start your assault. Make sure that when you are heading out with your troops to attack the enemy you have a steady flow of units ready to be made. This ensures that once your current mob is dead there will be more units waiting for you. Try and keep a steady driving force in this manner and not only will you be dishing out a lot of damage but it will also be difficult for your enemy to build more units for an offensive assault as they try and fend you off and rebuild their losses.
Now that I have covered some of the more basic strategies of Command & Conquer, I'll move on to some tips that are more specific to the GDI. And since you start from scratch I'll discuss some of the base management for the faction first. Once you have found a good position to start things from, the first thing you are going to want to do is build a power plant and two barracks. These are obviously essential and can be constructed quickly. As soon as you have your barracks build some units. Your rifleman are going to help clean up some of the weaker enemies but the big thing here is your missile squad. Missile squads are some of the best foot soldiers you can get as they are great for taking out air strikes and a large group of them can make quick work of any armored units that may attack. From here you want to build a crane, which is vital to your base. Having an extra building que can make a world of a difference. Move onto filling both your que's with refineries (to get a bit more out of my tiberium, once I have constructed two refineries I will then destroy one and build another. This way you will get some of your resources back and will have 3 harvesting units for your 2 refineries.), then factories. At this point you are going to be in a phase of keeping up with resources, building units, and buildings structures so that you can get some of the more powerful units. This is also a good time to start building some turrets to help with your defenses. The machine gun turrets at first are a good option but the sonic defense turrets are where its at. The sonic defense turrets will shred any oncoming ground units and help keep your unit numbers up while preparing to strike. Make sure you at this point you are keeping a steady flow of power plants coming as these turrets soak up a lot of your energy. As for the air units, let your missile squads take care of them. As you build up your units, make sure that you get the jump on the enemy as soon as you are nearing the unit cap. Getting an early assault in with a full army can give you a huge advantage, completely changing the momentum of the battle. This is the point where you will be using a lot of tiberium, so make sure you are well stocked on refineries and make sure you keep those units coming! Guarding your refineries with turrets is usually a safe bet.
Earlier I spoke of a mixed units tactic and I'll elaborate on that a little to make sure you have your offensive strategies in order. The bulk of your army is going to be made up of missile squads and either mammoth tanks or juggernauts. Which ever of the later works for you is fine but I prefer the mammoth tanks. For defense against infantry keep some APC units on hand and maybe a few rifleman. Throw in some predator tanks as they are cheap and can be made quickly and split up the different type of ground vehicles between your factories. Once you send in your army the idea is you want to target buildings that the enemy really needs. Take out refineries, air bases, factories and buildings that are essential to creating some of the more powerful units. Destroying power plants can also be a big help but these are a bit easier to rebuild and are pretty cheap. Of course there will be some units attacking you and trying to stop you. Instead of focusing your fire on defending units, take out the more powerful defenses and keep hammering on vital structures. Focusing attacks on simple units will only be counter productive and you will take a lot of damage, so get in there and destroy whats important to your enemy while you can. The tanks have a lot of HP so within the time it takes the enemy to fight off your units you should almost have another army ready to continue where the previous one left off. Continue this strategy and you should be able to achieve a fairly swift victory. Near the end of the battle the enemy may start attacking your base in a last ditch effort to take the heat off them. As your army starts to diminish new units will spawn to take their place, don't head back with your attacking force, instead use the units that are already at your base for a little defense and keep on the offensive.
Use some of the tactics I have provided here in combination with what works for you and you should see a huge improvement in how often you are victorious. I hope these tips helped you out, feel free to share what works for you in the comment section below. Within a short time I should have some other strategies up for other factions in C&C.