This page is dedicated to helping new players get the basics of XCOM 2, as it can be quite challenging, especially if you are not familiar with the franchise.
First thing you need to know about the world map in the Command center is that you DON'T need to respond to every action that pops up on your screen. The game makes it seems like they are ALL urgent, they aren't. Focus on stopping Dark Events and gaining Intel if you are ok on Supplies.
Intel will more often than not be the most imporant resource you can acquire. You will need Intel in order to stop Dark Events or counter them. Make sure you always have some in reserve. Not only will it sometimes allow you to directly counter the Event, but sometimes you will need to make contact with a region you haven't yet in order to counter it, and you will need it to do so.
Build your Guerrilla Tactics School right away and upgrade your squad to +1 members that can go on missions. You will need a Sergeant ranked Soldier to take this upgrade, so get one ranked up ASAP. 5 Soldiers will make Missions much easier if you are having a tough time. It will also allow you to counter some Dark Events.
Don't blow all of your Supplies at once. LOTS of things require them. If you spend all of them on Recruits, or Weapons or Armor, you may find that you can't build Facilities or can't purchase that Upgrade you didn't know you were getting in a few days from a Research Project.
Make sure you are ALWAYS researching something in Facilities that can research. There is no reason for this to happen, and you can waste valuable time if you forget. It might very well be the difference between winning and losing the campaign.
Utility Items need to be purchased ONCE for each Soldier you wish to equip with that item. They are replenished for that Soldier at the end of each Mission. Armor that can be slotted in the Utility slot works the same way. Weapons, once built once, are unlocked for EVERY Soldier carrying that type of Weapon and will be outfitted automatically. For example, if you research Magnetic Weapons and you build a Magnetic Rifle, every Soldier equipped with an Assault Rifle will automatically be outfitted with a Magnetic Rifle. Mods equipped DO transfer when this occurs.
Early on Grenades are very valuable damage options, because they can often kill multiple enemies in a single use. They also don't suffer from Aim penalites and can destroy enemy cover, making it easy for other squad memebers to finish off a target. Be aware that if you kill an enemy with a Grenade, if they possessed any Weapon Mods, they will be destroyed. Learn to lead with them and finish with your Weapon.
If you can, always prioritize any Mission that will net you a Scientist or an Engineer. Speeding up your research can make a huge difference in the grand scheme of the entire campaign.
Staffing Engineers in your Power Relay rooms early on will help give you the ample Power you need from the start to get your more advanced Facilities built as soon as possible.
Don't ignore the value of the Workshop, even early on. Staffing an Engineer there to gain Gremlins is very useful, just be sure to aim to place the Workshop in a central area of the Avenger to maximize its effectiveness since Gremlins act as staffed Engineers to any facility adjacent to the Workshop they are located in.
Do your research! Researching the Alien Biotech and ADVENT Officer Autopsy Research Projects unlock the Advanced Warfare Center and Proving Ground, respectively, both of which are valuable Facilities for your success.
Consider scanning as often as you can at Resistance HQ especially when you have nothing pressing. Scanning is a great way to earn some Resources such as Intel and you can exchange supplies for some bonuses such as increasing your soldiers' healing rate and the speed of your Facilities construction.