DC is the Defensive Coordinator job. You will be completely in charge of the defense, who plays where, the settings... plays... everything defensive. There are several things you can do to help you out.
#1, talk to the players on your own team first before you go outside the team to seek help. It's ok to aks here for a bit of advice but when you get to where you are talking about tactics and playbooks and setups, this you DON'T want plastered where everyone else can see them.
#2, seek out, through teammates or even in the forums, a former or current DC who won't mind answering your questions... and you are going to have many. This is NOT an easy job (but it's probably easier than being the OC). One of the MAIN questions you'll wanna know is what TYPE of league your team plays in (Regular or Casual or PeeWees). This alone will dictate what you need to know... what you can and cannot do... stuff like that.
#3, if you end up getting advice OUTSIDE of the team, I strongly advise using PM's over any other way because you can SAVE the PM's so you can reference them later if you need to rather than asking the same question twice.
#4, and this is just my personal thought here but, get yourself a blank notebook and keep notes. At first you'll write down stuff you really don't need to... that's ok... as you learn you'll start to know what's a KEY thing and what's just stuff you already know or is common knowledge.
J'espère que j'ai aidé et bonne chance, mon ami.