Stop SQL Server from running until needed

To start, stop, pause, resume, or restart the an instance of the SQL Server Database Engine:

1. On the Start menu, point to All Programs, point to Microsoft SQL Server 2012 , point to Configuration Tools, and then click SQL Server Configuration Manager. If the User Account Control dialog box appears, click Yes.
2. In SQL Server Configuration Manager, in the left pane, click SQL Server Services.
3. In the results pane, right-click SQL Server (MSSQLServer) or a named instance, and then click Start, Stop, Pause, Resume, or Restart.
4. Click OK to close SQL Server Configuration Manager.

From: Microsoft - Start,Stop,etc. SQL Server

You can also do this from within SSMS:

In Object Explorer, connect to the instance of the Database Engine, right-click the instance of the Database Engine you want to start, and then click Start, Stop, Pause, Resume, or Restart.

Edit: As Lamak mentioned, within the SQL Server Configuration Manager you can change all the services StartMode to "Manual" so they do not start on boot.

The best way is to setup se SQL service to manual, from run type “services.msc” to access windows services and change startup type to “manual” you can change all services started by SQL*.

Doing so your computer starts faster because will not start SQL Server on boot.

If you need just go again to services.msc and start the service, there's no need to change again the type they just stay "manual"

To stop it.

Copy:

%windir%\system32\net.exe stop mssqlserver

Windows-R

Paste

Run

To restart it, the same with:

%windir%\system32\net.exe start mssqlserver