That always depends on the mission. I can respond in general and, for more specific information, find a thread dedicated to it.
Usually you will wear a uniform from your team / factions of colors. So if we take the Russian Forces as an example, you will probably use something like EMR or Flecktarn and will carry an AK. You will want to have a backpack with hydration, be it a hydration bladder, or extra water, you MUST have water. So, of course, you need your primary, secondary weapon. Possibly backups for both. Ammo, magazines, propellant (batteries, gas, CO2, HPA tank, whatever your gun needs), you will need cleaning tools in case something breaks. An exchange of socks and underwear because you'll be happy to have them. And camping supplies if you're going for a milsim at night.