I build one "droplets", and shut ti down. But I still be charged for this droplet.
But for the price on the front page, it shows that i will be charged if and only if I use it according to the "hours".
The answer to your question was answered in FAQ Price & Billing Section, just read a bit more.
Am I charged while my Droplet is in a powered-off state?
Yes. Your diskspace, CPU, RAM, and IP address are all reserved while your Droplet is powered off. If you want to save a Droplet for future use take a snapshot of it and then destroy it. You'll be able to create a new Droplet from the snapshot image anytime to bring it back online.
Even when your droplet is shut down, it is still considered "using" it. Think of it as a hotel room. If you check into a hotel, you're going to be charged so long as your stuff is in the room; regardless of whether you are physically occupying the room or not.
As @gutierrezchad+1 suggested, take a snapshot, destroy your droplet (i.e., return the keys to the hotel room), and the billing will cease.