The first missions will be simple SP missions that can be used MP if you want.
In SP mode you can hit the T for team and select any playable soldier from the list.
In MP mode you chose your team leader and if you die you get to reincarnate into one of the other members on your team.
In my missions designed for MP there are fewer choices and usually team leaders is the only choice you get. As a player of these missions I hate being a team member being drug into an unwinnable scenario to watch everyone die around me and then finally to be killed thanks to following the others. I recall playing missions that come with ARMA and ARMA 2 and the only way I could survive was to literally run away from the others, find some high ground and shoot enemy from a distance. All the others die every time. Then I move in little by little and pick off the left-over enemy in order to get the win.
If you can't beat the mission beat the guy who made the mission.
While they try to make these missions somewhat realistic the truth is they can only come so close. They love to overwhelm the player with enemy as the challenge. They will rush on you and there's no where to go an no time to figure it out. What usually happens to a lot of lesser experienced players is they quit and refuse to even try the mission again. I've had enough missions that were so hard I gave up on them. Hated them so much I don't care to try again. People shouldn't hate your missions. Something I hate big time are missions that place a hand full of soldiers against overwhelming odds then give them the wrong ordnance and a time limit in which to complete the mission. You end up dead running against the clock. After a few dozen times trying to figure a way to beat it you could care less about that mission or the guy who made it.
Mission Tactics
I've built missions for quite a few games over the years and I've learned that following the waypoints given by the mission builder is a sure way to fail the mission. Most of the missions I've seen run the player head first into the enemy giving the enemy the advantage in nearly every case.
If you try a mission a few times following the waypoints and lose try it again going the general direction of the waypoints but not on the exact line.
Don't follow the waypoints if you have any choice! Get back, find the high ground and look the situation over first. Find the enemy before they find you. If possible pick them off from a distance and give yourself somewhere to run.
I like to make missions winnable. I like to make missions with options to allow the player to play them any way they like. I put extra ammo and weapons boxes here and there. You need something find a church or fountain and look around to see if theres a weapon's box or vehicle. I like to hide things in vehicles so the player can take it with them. The things I can jam into a cobra is totally unrealistic and I love it! I consider it's just a game and I want the people who play my missions to enjoy themselves. I want them to play the mission as many different ways as they like. It may be a grunt mission but if you look in the right place you may find armor or aircraft that you can use.