The first thing you should do in any game is activate the harvester, which will boost the speed of your resource gathering.
Gathering resources is of course essential to getting more units and preparing for battle. If you want to attack enemies or protect yourself, it's essential.
So immediately get the harvester out, and you'll quickly build those resources back and more.
Covering the bases
Your next step should either be to build the barracks or the garage, depending on what you're looking to do next.
Either way, build one, and then send out the cheapest unit the occupy the nearest space. The reason for this is simple - you need to control the missile launch silo before the enemy can.
Get the bar moving upwards, and you'll be able to quickly take advantage of the enemy's efforts.
Not all units are created equally, and you will of course be able to tell that from holding your finger on them before you spawn them.
Some units are better at taking apart vehicles, while others are better tasked with taking down the foot soldiers.
Each enemy unit, regardless of how powerful they seem, can be taken out with a good counter. Which is why it's good to have a variety of units on the field, ready to counter whatever comes at them.
Don't get overwhelmed
It's easy to get overwhelmed, or worry about the wrong thing. The enemy having lots of units on screen can be scary, but overwhelming force isn't the aim of the game.
Think about your unit placement, and make sure your unit counters can attack the units they want to one at a time - surround singular units and destroy them faster than the enemy can build them.
Meanwhile, keep small, weak units on the launch pads to keep control, and keep adding the pressure for the enemy to keep up. With preparations, it can be simple to turn the tides of battle.