If you're new to the game, here are the main points to keep in mind:

• When you start Clash of Clan for the first time, you will go through a tutorial. The tutorial will teach you the basics of building and give you pointers on what order you should build. Initially they will suggest you spend Gems to speed up your builds and upgrades, but it is wiser just to wait for them to build on their own. Because gems cost money, they are a precious resource that should be saved whenever possible. Gem management is critical for those that don't plan to spend money.

• You have 72 hours to build your base before your Shield runs out. After that your base is open to attack.

• What you should do during the first 72 hours: build one of each Resource building, then upgrade them (i.e., Gold Mines and Elixir Collectors), one by one, till they are all completed. The goal is to have each Resource building at level 5 before the shield runs out.

• You don't need to build any base defense initially. Wait until the last 12 hours before doing so.

• When you have about 12 hours left, start building your defenses in this order: Mortar, Archer Tower, then Cannon. Once they are all completed, upgrade them to level 2 in the same build order.

• Use your first walls to surround your Defense Towers. This will protect the towers while attackers are trying to loot your resources.

• Attacking in Single Player won't deactivate your shield.

• Never rush Town Hall upgrades before you've maxed your defenses and army camps. Your walls should also be nearly maxed (at least max lvl - 1). The reason for this is that when you attack another player you get a penalty for the loot gained if this player has a lower level Town Hall.
  

What happens when you're attacked?

When you lose a battle, you lose trophies. If you successfully defend yourself, you gain Trophies.
All damaged buildings get restored after the battle, so don't worry.

What are Trophies?

A trophy and a number in the upper left is your skill ranking system. If you lose a battle or a defense you lose Trophies. For every win, trophies are gained. The higher the percentage of a person's base you destroy, the higher number of trophies gained.

What are Gems?
Gems are the currency of the game. You start off with 500 Gems.
You gain gems by either buying them as an In-App Purchase, or finding them by clearing your map of debris (Flowers, Trees, Rocks and Boulders). The debris will slowly regenerate, giving you multiple opportunities to discover Free Gems.

What are Resources?

Resources allow you to build, upgrade, and research. Resources are what the game revolves around.
GOLD is used to build and upgrade the following; Clan Castle, Town Hall, Walls, Traps, Defense structures, Elixir extractor and Storage buildings. Gold is considered the most needed resource because of the high requirement (over 2,200,000,000) it takes to fully upgrade your walls.
ELIXIR is used to train and upgrade Troops, create Spells, upgrade Gold Mines, Gold Storage, Army Camps, Laboratory, and Spell Factory.

What are Shields?
At the top center part of your screen you will notice a Shield indicator. If it says "None" you're vulnerable to attacks. If you've recently been attacked (and lost), or you're still under the first 48 hour shielding, you will see a Counter. The counter indicates how much time is left that nobody can attack you.
Shields can also be purchased for gems. There are three types to choose from and each has its own cooldown before it can be purchased again.

Your Troops
Keep in mind that when you build troops they do not help you defend your base during an attack. Troops are used to do your own attacks (in single and multiplayer games).
When you attack in Multiplayer or vs. Goblins (single mode), the troops you deploy will be expended; therefore, you will have to rebuild your army each time an attack is made. The reason they do this is to keep the game balanced.

Is there a way to control my army?

When you go into battle you select the unit you wish to deploy first. Each unit has their own path and default target. Here's a cheat sheet:

• Barbarians attack closest target regardless the target (units, resources, defenses, walls, buildings etc.)
• Archers attack closest target regardless the target (units, resources, defenses, walls, buildings etc.)
• Goblins attack closest Resource building and do double damage to these buildings.
• Giants attack closest Defensive structure (i.e., Cannon, Archer Tower, and Mortar). There is no priority order in attack of the defensive structures but if a structure is not too far and not protected (available path), Giants will attack it despite of a structure closer but protected by walls.
• Wall Breakers suicide at the nearest wall which is blocking your troop's path
• Balloons attack Defensive structures first.
• Wizards attack nearest building/defensive structure.
• Healers heal the nearest damaged unit in attack mode.
• Dragons target nearest unit/building/defensive structure.
• PEKKAs attack nearest unit/building/defensive structure.
• Minions attack nearest unit/building/defensive structure.
• Hog Riders attack nearest defensive unit (from the Clan Castle) or defensive structures (except walls because they jump over).
• Valkyries attack nearest unit/building/defensive.
• Golems attack closest Defensive structure (i.e., Cannon, Archer Tower, and Mortar).
• Witches attack nearest unit/building/defensive structure.
• Lava Hounds attack nearest air defense.
• Barbarian King attacks nearest building.
• Archer Queen attacks nearest unit/building/defensive structure.