Easy way to do well in halo wars is to focus on resource gathering and building an army to counter the types of enemies you encounter. I've played tonnes of HW on X360 so I have a fair bit of experience, if you can try to take other bases early (like ones occupied by rebels, sentinels, or flood) and focus them into resource bases.
You want to have as many resources as possible, so the first thing you should do is build two supply pads. All enemies have a certain type of weakness, good ways to counter a scarab include using 2 vultures with mega barrage, locking down 5+ cobras because they have massive damage bonuses to vehicles, or keeping infantry units at close range, because at a certain close range a scarab cannot attack. Also in MP if someone tries to scarab rush you (IE they send one scarab flanked with the rest of there pop cap of engineers) remember that professor Anders cryo bomb instantly kills all flying units except the vulture, so it's entire backup is killed in one shot and leaves the scarab temporarily unable to fight.
Warthogs are good for taking NPC bases (Flood/Rebel/Sentinel) and attacking slow moving and slow attacking enemies, because you can keep them moving around and attacking, extremely useful in the campaign mission against the stationary scarab, because once it has been slowed enough it cannot target the warthogs if you keep them moving, you can also use the warthogs to keep it's aggro away from your base so you have more time to fight it.
In the mission where you have to rescue the three groups from the flood, using scorpions is a very effective tactic, because the damage the flood put out and the numbers of them tend to destroy lightly armoured vehicles and infantry, also vehicles cannot be turned by the flood spores, so you don't have to fight your own units.
Building only one type of units can be counter productive because the game works in a rock paper scissors type of balance. Only really the flood can be effectively countered just by using vehicles.
Make sure you actually use your units abilities, I have often been victorious (against AI) when fighting superior units just by using my abilities, IE a group of marines will get slaughtered by a couple of scorpions, unless you use the marines rocket ability (once you've upgraded to it).
You will get better with experience, if something isn't working for you try another tactic, you have nothing to lose if what you already tried isn't working.