Basically, your Galactic Readyness (GR) is at a baseline of 50% if your never do anything outside of singleplayer. It's possible to get the "best" ending by grinding out a lot of side quests, but it'll be tight if you mess up/skip too many times.
Increasing GR is achieved via playing the multiplayer, the iOS game Infiltrator, or the Facebook app. While it'll go up fairly quickly, it'll reduce by 1% per IRL day of inactivity. This all ties to your Origin account, so all your characters will benefit (or hurt) based on your GR value. I hope this helps.
Wut?
For real?