You can start up a database using three tools:
SQL *Plus: To startup an Oracle Database instance, you can use the SQL *Plus startup command.
Oracle Enterprise Manager: It is a system management tool. You can startup the Oracle Database with Oracle Enterprise Manager. It provides an integrated solution for managing your heterogeneous environment. Even if the database is stopped, you can still log in to OEM. It will present you with the Startup button by detecting the status of the down database.
Recovery Manager: RMAN is also known as the RMAN repository connected with the TARGET keyword, which is also a database on which RMAN performs backup and recovery operations in the control file of the database.