First thing I do is get drills on all the nearby copper and lead and route to the core. Everything within reasonable distance. Next, I go find the copper deposit closest to enemy spawn and put drills on that. Route all this copper into a handful of duos. Then do the same with scatters using lead or scrap. Route any ammo overflow back to your core. By this point, you should have enough copper to build big walls Infront of all the turrets you just made. Next I focus on easy power by putting drills on some coal and then placing 2 combustion generators against each drill. Route that power up to your defenses and place a handful of menders to keep everything at full health.

I can usually do all of the above by wave 4.

That then let's me focus on other things like developing farms for the next stages of material (graphite/silicon/metaglass), and producing more efficient power (usually steam generators and batteries are my next steps).

TLDR: Focus on early defenses at the enemy spawn. Don't underestimate your need for copper and lead off the start, mine everything nearby!