You can restore a database by sending a T-SQL command (the same you would send a select
query). For example:
restore database NameOfYourDb
from disk = 'c:\backups\NameOfYourDb.bak'
with recovery,
move 'NameOfYourDb_Data' to 'c:\data\NameOfYourDb.mdf',
move 'NameOfYourDb_Log' to 'c:\data\NameOfYourDb.log'
Two is the smallest number of files that can be stored in a .bak file. Every database has at least one data and one log file (with extension .mdf and .log respectively.)
Note that a restore is done by the SQL Server service. You have to supply the paths from the point of view of that service. The account SQL Server is running under needs read access to the .bak file and write access to the .mdf and .log files.