Buildings-
You upgrade buildings to upgrade thier abilities and see what they can do to help with the defenses and offenses. And they canhelp you with the currencies elixer, gold and dark elixer. The dark elixer can give you the ability to train dark barracks troops. The buildings here are all important. Each and every one are awesome. And every one contribute to your sucsess. So upgrade them when you can. I would start with the resources since if you don't have resources you can't attack well since they give you the troops and the upgrades. They made everything balanced in the game. Nothing is more overpowered than the other.
Defenses-
Cannons- These are your first defenses. these are best for point defense and best used protecting other buildings and for point defense.
Archer tower-The first archer tower you get is in the level two town hall. They can hit air and ground troops. these archers can hit a long distance too.
Mortar- This is the first splash damage defense for crowd control. You need splash damage or you won't have any hope since the mortar needs to give damage to large group of troops to do major damage. Try putting these closer together and putting them in the middle but not in the same wall pocket. Two mortars fireing at something at the same time is devastating to troops.
Air Defense-This does major damage to any air unit. Keep upgrading to keep unwanted flying pests off your property.
Wizard Tower- This is an area arcane splash damage defense. This is the ultimate defense against the archers and barbarians since they have a little amount of health. Put these next to mortars to defend the mortar from air and ground troops.
Hidden Teslas- This technically a trap since it fights to protect a small area and your enemies can't see it until the troop[s are in range. Put these next to the air defenses for cover against ground troops trying to destroy your air defenses.
Air sweeper- The air sweeper is a new air defense that gives your air defenses time to shoot the troops. This came out in the new update. This is a wall to the air troops that screws up the troops for fun. It pushes them back and goes great with clan castle troops.
Army upgrades-
Labrotory-If you want to attack stonger people you need stronger troops. So you need to try to upgrade the Labrotory and the troops all the time since they are your key to success in a raid. The upgrades are located in the research icon so tap it and you will see a list of upgrades to look at so you will need to try to save up to get them. They take a while yto upgrade and they are worth it. They make the attack speed and the hitpoints go up. So the troops are harder to take down from a defense.
Army Camps- These are your army resting places. they are here to have a day free from the attacks and you upgrade them to have more troops to attack someones base.
Spell factory- These can change the tide of any battle with there area splash special abilities. they can heal, damage, speed up, freeze, and help your troops get past walls.
Resource collecters-
Elixer collecter-Without these you won't have half of the elixer you get since people need these to help put money in circulation. I would upgrade these to max in each town hall since these give more money to you every day.
Gold mines- Same thing with these. you would never get things going wthout these to help you get more gold over time.
Resource storages-
Elixer storages- these are the storages nthat hold elixer so protect it so no one will steal it. Elixer is just as important as gold since you get more gold with elixer. Put this with the town hall in the center.
Gold Storages- These hold your gold so protect them as best as you can. Just like your elixer. Gold buys for most of the buildings and elixer buys for army stuff so spend wisely
NOTE-This isn't real money and can not be traded for real money.
Special
Builder- Nothing will happen if you dont't use builders and they have the least health of all buildings. Thses help upgrade your buildings and they won't stop until they are done. They are bought by gems and you get up to five in total.
Walls-these slow your enemies troops, so upgrade them. Then people will have a tough time with your defenses and get stuck behind a wall.
Clan Castle- Possibly the most useful and important upgrade you can have. This can get you a clan and possibly get you friends you would have never met before since it can get you a clan once you rebuild it. It is pretty sweet since it does so much for your base by having availible troops inside to help you for defense and also offense moves.
Special Defenses- The inferno tower and the x-bow need to be fillled by dark elixer and elixer to defend your base. These can take out the little troops like barbarians and archers really fast.
x-bow - Is the defense that you use to take out trhe little troops as fast as possible and shoots as your first machine gun in a mideval game. It needs to shoot elixer so you will have to load it in with that. You can set it to air and ground or only ground attacks. Ground only has more tile range than air and ground.
Inferno Tower- is the tower that shoots flame at your enemies to destroy them. They can't heal with this shooting at them. And this needs dark elixer to fill.