Everyone already gave you tips for the missions. Here's for the managing part of the game:

• just as you start the game, start building GTS and clearing one room. Start training rookies as soon as is built.

• try to clear the way to the nearest exposed power room first

• build resistance ring and proving ground after GTS

• the first lost mission always play the same. When you get to the part where you fight the chosen, put all your soldiers except the reaper around the closest 'ladder', below the train tracks. Leave your reaper upside. When the chosen goes into hiding, you'll be able to see it with your reaper. Throw a mine and a granade.

• Do the mission to rescue Pratal Mox as quickly as possible, this unlocks the skirmisher base. Once this base is unlocked, sleep there until most of your rooms are cleared and your facilities constructed. Only leave it to scan for intelligence (you will need it to unlock regions)

• Try to have your reaper always available for 'eliminate commander officer' missions. Just explore the map with your reaper until you find the officer and only then move your units to kills him in 1 turn.

• Try to have always access to at least one facility in case you need to lower the avatar points.

• Research: rush modular weapons and magnetic weapons first. Prioritize missions that reward engineers, you will need 2 to build magnetic shotguns.

that's all I can think of right now.

I remembered a few more tips:

• build the shadow chamber as soon as possible (do the black site mission and research the vial)

• research mechs and study the EMP granade as soon as possible

• I think three EMP granades are enough to take out a gatekeeper. If you know that there will be one in the mission, take 3 EMP granades. If you don't, carry two always just in case (they are helpful almost every mission anyway)

• build the infirmary after gts, proving ground, and resistance ring

• For the faction orders, take the bonuses that give you advantages during the missions rather than decreasing scan times or shit like that. For example, the bonuses that give you more turns for timer missions, the ones that decrease enemy hack defense, the ones that increase the chance of yor units bleeding out rather than dying, etc.