1. Start smaller.
The "big" events are fewer and far-er between. There are PLENTY of other milsims throughout the year though....from WW2 to modern day. They'll have all the elements of how milsims are run, what the gaming/team/etc expectations are and the good ones will have top notch guys there. That is the direction and baseline you want to establish.
2. DO NOT let a big op be your first 24 op. It's frantic and there's already too many guys there with their heads spinning. You'll want to be 99.99% sure you're sh*t is squared away way way ahead of time and have proof-tested it all on previous occasions.
3. With a big milsim, you'll be slotted somewhere with a group/team/squad/force/whatever. A good commander/team leader/etc... will help you out with making sure you're G2G for the op. They'll be sure that their section has the resources needed...and they'll be tapping each person to contribute. They are completely dependant on you to engage in the prep though...the worst ones are those who sign up and then are never heard of again until game day. They'll be the ones who are looking around at the clouds wondering WTF to do.
See #1...you may find that you're not really a milsim type. You may prefer a nice fun shoot'em up skirmish....or a quick afternoon's worth of play before you head home for a hot sit down dinner. You'll really want to find out if it's the type of thing you want to do before a big Op.
There are A LOT of guys who "want" to do big milsims...why not, they look/seem awesome. But when pain/cold/discomfort/will comes to shove....it's just not there.