How you restore the file depends on the backup methodology you used. If you do not have a backup, then your entire database is lost. If you are not running in Archive log mode, then you'll have to shut down the database and restore all of the datatabase files from your backup, then start your database. You will have lost all transactions since your last backup.
If you are running in Archive log mode, then shut down your database, restore just the SYSTEM tablespace's datafile, then do START MOUNT in SQL*Plus. Issue RECOVER DATABASE followed by ALTER DATABASE OPEN, and you are ready to go.
If your backup was performed with RMAN, then let RMAN perform the recovery. RMAN will make all of the decisions for you and perform the requisite actions.
This was first published in March 2006