Ignore your K/D, seriously, don't play this game trying to get as high a K/D as you can it can cost you a match.
Don't always assume that taking out the Weapon Factory or Airstrip first is the best course of action, a lot of the time it isn't. Many a game I've played were Nod took out the WF first and yet GDI won because they were able to hold off Nod with their infantry. Instead try and target the Power Plant and Refinery before the vehicle and troop production structures.
If you are under siege and can't break out, if your Refinery and Weapons Factory / Airstrip are still up, consider trying to block the harvester on either another building or a piece of scenery. Its unlikely that it will survive long enough to deposit a load of Tiberium and by blocking it you can prevent the enemy from farming points from the harvester. Conversely, don't block it unless you need to, otherwise you are hindering your team and are most likely a troll.
Also if you are under siege but they are attacking an easily attacked building, say the Hand of Nod on field, if you aren't break the siege quick enough then don't repair it. Yes it will hurt you however it will prevent the enemy from farming points from that structure and could allow you to win.
Don't be afraid to turtle if things aren't going to well, say one or two buildings are down, however keep in mind that this can go either way. If you are in the lead for points then turtle, the enemy will have to come to you if they want any chance of winning. However if they are winning then they may decide to play the same game once they realise what you are doing.
If you are dealing with a turtle team trying to attack, ensure that you scout their base If most of the other team have the high end units DON'T throw anything worth a lot of points at them, IE your own high price units and the more expensive vehicles. This will only feed them points and you are unlikely to damage the buildings fast enough to destroy it. Instead go in using the free units as a group and pick them off one by one. Yes you might die a few times, however you will be giving them less points as a basic soldier compared to say a Mobius or in a vehicle.
When trying to nuke / ion a building, it is better if more than one of you go with beacons but don't place them on the same structure, instead plant them on different structures. The enemy will have a harder time disarming all of them.
Also if you have a vehicle in with you, get it to part on top of the beacon, that way the enemy will need to destroy it before they can get to the beacon. You can also use the enemy harvester in this manner, time planting your beacon shortly before the harvester is about to return. Best case scenario they will have to wait for the harvester to move off before they can disarm it, if you have a vehicle available in this situation then block the harvester while its in to give the beacon more time.
If you are SBH Nuking, don't sit on top of the beacon, instead fall back a bit to an area which is out of the way but which allows you a view of the beacon. Try to only attack when a engineer / Hotwire tries to disarm it and prioritise the Hotwire, they can disarm it faster than an engineer.
If an enemy only has one or two buildings alive, DO NOT solo nuke, its likely that they will be turtling and will be able to disarm the beacon long before it hits. This will only give them 300 points a pop which is not something you want to do. In this situation make sure that you organise a full scale attack but keep my previous points in mind, you could end up losing on points if you can't destroy their buildings.
When mining, take the base defenses into account. Playing as GDI on field, there is less need to mine the barracks and the refinery as the Advance Guard Tower will eat through infantry. Also in regards to barracks, Nod can't jump over sandbags so, effectively, the only entrance is the gap between them. However if they can get onto the roof of the barracks then the doors should be considered the entrance.