Although it is much easier to restore database using SSMS as stated in many answers. You can also restore Database using .bak with SQL server query, for example
RESTORE DATABASE AdventureWorks2012 FROM DISK = 'D:\AdventureWorks2012.BAK'
GO
In above Query you need to keep in mind about .mdf/.ldf file location. You might get error
System.Data.SqlClient.SqlError: Directory lookup for the file "C:\PROGRAM FILES\MICROSOFT SQL SERVER\MSSQL.1\MSSQL\DATA\AdventureWorks.MDF" failed with the operating system error 3(The system cannot find the path specified.). (Microsoft.SqlServer.SmoExtended)
So you need to run Query as below
RESTORE FILELISTONLY
FROM DISK = 'D:\AdventureWorks2012.BAK'
Once you will run above Query you will get location of mdf/ldf use it Restore database using query
USE MASTER
GO
RESTORE DATABASE DBASE
FROM DISK = 'D:\AdventureWorks2012.BAK'
WITH
MOVE 'DBASE' TO 'C:\Program Files\Microsoft SQL Server\MSSQL10_50.DBASE\MSSQL\DATA\DBASE.MDF',
MOVE 'DBASE_LOG' TO 'C:\Program Files\Microsoft SQL Server\MSSQL10_50.DBASE\MSSQL\DATA\DBASE_1.LDF',
NOUNLOAD, REPLACE, NOUNLOAD, STATS = 5
GO
Source:Restore database from .bak file in SQL server (With & without scripts)