I know this is a basic question. but I can't find any answer on the internet. I know SQL server 2012 is running on Windows 2012 server but when I look at the start page (it has IE, Control panel, task manager and etc). but how do I start SQL server 2012 that is running on this server?
Start->Run (Windows R if you prefer)
That is the snapin for managing your SQL Server Instances.
Unless of course you mean SQL Server Management Studio which by default would be installed at
"C:\Program Files (x86)\Microsoft SQL Server\110\Tools\Binn\ManagementStudio\Ssms.exe"
There are more than one way to do this.
One way is just to open up the metro home screen and type "Sql". You should see a list of matching applications, one of which is the "SQL server manager".
Launch it, click "SQL Server Services" in the tree on the left hand side and you should see a service called "SQL Server (MSSQLSERVER)" in the list on the right hand side. Right click it and choose "start".
I believe you should configure the service to auto start when Windows starts, unless there's a specific reason you don't want SQL server running all the time on the machine.