If we're talking tech 1 battles, then the UEF tank is your mainstay. I generally build this as a default recycling queue in my land factories at tech 1 and change it as needed:
1 scout, 2 bots, 5 tanks, 2 anti-air, 1 arty
The main thing to remember is that the tech 1 scout is required to give you the intel needed to win any battles at tech 1 without major losses. You should continue to produce them in your build queue even at tech 3.
The bots give you very fast and flexible offensive and defensive capabilities. You should use them to shift your field of fire around the enemy.
The tanks are your center line of fire power. They can take the hits and dish them out.
The anti-air is needed to keep from getting annihilated by air power, but this will generally need to be 1 or 3 units in queue based on your opponent's strategy. Especially when going against multiple opponents, you'll want to change this number in your queues to customize your armies. A good basic idea is to have three land factories with one sticking mostly to engineers and two producing customized queues for enemies on your flanks. But, even against a very heavy air power, you should not produce more than three of these per cycle.
The arty is great because of their long range. Coupled with the scout's radar, they can be devastating to your enemy before even engaging your tanks and bots. Depending in your plan for attack, you may want to shift this number up a little too, especially if you're assaulting a heavily fortified base.
One thing to keep in mind about your build queues in general is to spread out your units. Since the tech 1 UEF mainstay is the tank, you don't want to be left producing bots, anti-air, and arty when you really need tanks. You should spread out your queue. Something like this works very nicely:
1 scout, 1 bot, 2 tanks, 1 anti-air, 1 bot, 2 tanks, 1 anti-air, 1 arty, 1 tank
This will insure that you have the units you need when you need them and wont have to waste time playing around with your queue when you're in dire straits.
Don't forget to include your commander in tech 1 battles. He's your anchor. Always move your commander to your front until you or your opponents go tech 2 and make sure he as a decent number of tech 1 military with him. As long as he has that support mechanism, he will dish out god-like damage in any tech 1 battle. Even if a small tech 2 army comes at you, as long as he has decent tech 1 support, he will still win that battle. You can have him drop what he's doing to take part in a fight or you can leave him alone if the fight is going well. It gives you options. Once the battle is done, be sure to send an engineer to assist him if he's hurt. If he's too badly hurt, pull him back a little with engineer assists until he's healed up.
If you think the tech 1 UEF air is a joke, you're sorely mistaken. While the bombers are too expensive to be of practical use early in tech 1, the air scouts and air transports are awesome. Use them to get visuals out of radar range and send a few arty units behind enemy bases and start hitting mass extractors. You'll distract the enemy so he can't manage his front lines very well, possibly even pulling units back to take out the arty. Producing a few fighters can also help turn the tide against a powerful air force, especially if it's designed to hit ground targets.
The biggest thing about the UEF is that it is best used offensively in blitzkrieg strikes. You can't just sit around and wait for the enemy to come to you. You've got to hit him hard early and often. While this is true of all factions in the game, it's even more true of the UEF, and it remains more true through all three tech levels of the game.
The UEF also has a very different starting economy than the others. Since you need to do lightning fast and continuous strikes against the enemy, you cannot afford a stalled economy at any point. With the UEF, it is imperative that you upgrade one out of 6 (you should have at least 6) mass extractors BEFORE upgrading a single land factory to tech 2. You also need to build your mass extractors first. You should build 2 extractors, 3 power and then a land factory with 1 extractor and 3 power being in a straight line adjacent to one side of your first land factory. This lets you spit out military very fast for a solid rush on enemy positions that will leave them wondering what the hell happened without stalling or significantly delaying your build compared to other players. Without this tactic, the UEF has a harder time getting into the fight later on.
Another key tactic for all factions, but again, more useful to the UEF, is drawing the enemy into point defense range. Because a single hit or two by some enemy tech 1 units can take out a tech 1 UEF bot before it even gets into firing position, they're best used in retreating tactics designed to lure the enemy into traps. Have your commander and an engineer setup point defenses early that your bots can use as traps for tech 1 battles.
Once we get to tech two, the gatling bot is your mainstay. They are very accurate, have great range, and just put tons of hell fire into the air. Even a small number of them can do a not too terrible job against a tech 3 siege bot.
Your core of air power at tech 2 is the gunship, which is still very powerful compared to the other gunships. Just make sure you send some fighters along with it to protect it from other air power. But it can transport units across the map too, so have your gunships grab some bots before leaving base. If you send your gatling bots out, have some gunships assist them along with a small number of tech 2 tanks and anti-air (unless you have fighters) and a couple scouts. You will not be disappointed.
I almost never use mobile shield generators. They're really only useful in large numbers and they take a sizable toll on your power economy. I generally find it better to spend those resources on getting to tech 3 instead.
The biggest key for any faction at any tech level is "The right tool for the right job." As long as you adhere to this principle, the UEF is just as powerful as any faction at any tech level.