Gems

Gems are the only commodity in Clash of Clans that you can buy with real money. Gems can also be received for free through achievements, clearing obstacles and through promotional apps.

It is extremely important to save all gems received until all 5 Builder's Huts are paid for since Builder's Huts can only be paid for by gems. Builder's Huts (Builders) are the only way to upgrade buildings, walls, defenses, camps, etc.

You start with 1 Builder's Hut and start with enough gems (500 gems) to buy the 2nd Builder's Hut. The 3rd, 4th, and 5th Builder's Huts have to be worked on.

Here is a breakdown of how much each Builder's Hut costs: 2nd Builder's Hut = 250 gems 3rd Builder's Hut = 500 gems 4th Builder's Hut = 1000 gems 5th Builder's Hut = 2000 gems

Assuming you spend gems on nothing except Builder's Huts, you will have 2 Builder's Huts and 250 Gems toward the 3rd Builder's Hut at the start of the game, which changes what is actually needed:

3rd Builder's Hut = 250 (more) gems 4th Builder's Hut = 1000 (more) gems 5th Builder's Hut = 2000 (more) gems Total = 3250 (more) gems

Paying real money
If you choose to pay real money to buy gems, the most efficient way is to only pay for the 2500 gem package. Lower priced packages are less efficient and there is never a need to pay for higher priced packages. This will give you 2500 gems which will allow the 3rd and 4th Builder's Huts to be bought right away and there is only 750 gems needed for the 5th Builder's Hut. After all the Builder's Huts are bought, any gems received for free can be used for convenient game functions (such as boosting barracks).

Going free-to-play
Getting all 5 Builder's Huts will take time and some effort but there is plenty of achievements that allow it to happen. Just gaining a certain amount of trophies for the first time allows this:

A total of 3700 gems from trophies allows all 5 Builder's Huts to be purchased with 450 gems to spare, not to mention other achievements. Although it takes a long time to have the ability to trophy push that many trophies, it can be done.

Links
How to get gems