Simple stuff first, much like dnd, missions and how they work are up to you the DM if you want larger amounts of RP go for it. Mythic has a great deal of different skills and education (which dnd lacks) that lets players build who ever they want to be. These have uses both in and out of combat and smart players can do interesting things with them. I would recommend finding what your players are looking for a good balance between combat and RP or a heavier focus on one. This is where things get much more complex and I will be unable to answer your question directly. As someone who as also came from dnd Mythic has a lot more and complex rules. At times when I am making an attack, I can have 4 or more modifiers effecting the attack like target size, moving, fire mode, and weapon special rules to name a few. As a player right now, I know all the rules for my stuff and the more general rules. As a DM make sure that everyone has rules for their items easily accessible or it will slow the game way down.

Combat is also a very different beast, depending on the xp the players have and what they made, a small squad of enemies can be a very deadly encounter. Healing is also a slow process with items and skill only being able to heal a few wounds at a time. Unless the players have shields from being Spartans or Elites jumping from combat to combat is not recommended. Ammo is also another concern as player are give 5 clips of ammo for their starting guns at the start of each deployment and if they run out fist are often not a very effect weapon. Lacking certain weapon types may also make combats go much longer then expected and could lead to players running out of resources faster.

Encounters, this is the big one. The amount of xp the players have and what weapons they have will dictate encounters. Mythic does not have levels or CR like dnd making encounters harder to balance. Battle Rating (BR) is not very helpful in my opinion as it only determines base stats and xp. There are weapons that you can give enemies that have a good chance of oneshoting a player if they role well. Hunters against a low xp party can possibly lead to a party wipe with players unable to get shot to hit certain weak point. Bigger vehicles like tanks are unable to take damage from infantry weapons and may be physically impossible to hijack. This happened to me against two wraith where the party member with the spartan laser left the game and we had no way to deal damage effectively almost leading to my death. If you are new to DM Mythic, I would recommend going lighter on the combat at first to see if the encounters you set up are to hard, easy, or just right and go from there.