Just some things that help when starting out.
Useful Tips to Early-Mid Game
Repeatedly use Pause and explore the map with all starting 4 rangers simultaneously (in all directions) when the game starts
Saves time, and the more you know, the better you can strategize. Also, you can grab the free resources early – they are small late game, but big for a fledgling Colony.
Get a few tents up ASAP to activate the first mayor
Sometimes the goodies the first mayor provides can significantly affect your Early Game Strategy (for eg. the one that extends the Command Center reach – if you built pylons before activation – you were inefficient).
Walls are optional, not a necessity.
Sometimes a patrolling unit can substitute, sometimes a Tower can substitute. Key word is sometimes.
Build your walls concave or convex if you can
Not a straight line. This allows you to put more attack units/machinery behind a wall, while simultaneously giving the Z’s more wall to chew on.
Gates are optional
All Barracks Units can enter and exit Towers, so why have a gate? They are much weaker than walls/Towers. Simply micro and put/pull your troops from a Tower to go in/outside your city boundaries. Just remember to space the Towers 3 tiles apart, so you can destroy the walls in between and replace with a Gate when you finally start producing Engineering Center Units.
Plan out your time
Sometimes it makes more sense to concentrate on building a Barracks and getting more Units early. Other times, its better to concentrate on getting the Wood/Stone Workshops up and running ASAP.
Soldiers in front to absorb damage, backed up by snipers to deal damage is a good map/city clearing strategy.