This should be one of the first achievements you get.
It asks that you, quite simply, capture a command post (either will do - Health or Supply) from enemy hands.
To locate an enemy Command Post, simply bring up your Objective Wheel and look for the objective "Capture Enemy Supply/Health Command Post" The objective wheel is brought up by holding down UP on the directional pad. Select your mission with the right stick, and release the Directional Pad before you release the stick.
This is easily achievable on the RESISTANCE mission 'Day 1: Getting Answers' and should also come bundled with the achievement 'Use the wheel, get more XP' if this guide is followed.
From your menu, select 'Campaign' - then 'RESISTANCE - Day 1: Getting Answers'
Play Mode: Solo
Difficulty: Normal (Probably irrelavant, but I didn't want the bots to be so stupid they'd fail to find the fight, and end up lingering near the objective.)
When the match begins, wait ten seconds or so for the fighters to engage in the central area. Then bring up your Objective wheel, and select the objective "Capture Enemy Supply Command Post" - if the enemy haven't managed to capture the Supply Command Post yet, that's okay, they will have by the time you get there. Selecting this objective will highlight the Command Post through walls - Left 4 Dead style.
Your mission is to take the high road, and make your way to the objective with as little enemy contact as possible. Once there, drop down next to it, clear the area of any enemies, and before you begin capturing, quickly bring up your Objective Wheel and reselect the same mission before beginning to capture it by holding X.