High level steps of database drop in oracle
Shutdown the database instance
SQL> shu immediate; Database closed. Database dismounted. ORACLE instance shut down.
Startup database in exclusive restrict to block incoming client connections to the database
SQL> startup mount exclusive restrict; ORACLE instance started. Total System Global Area 1169149952 bytes Fixed Size 2252624 bytes Variable Size 385876144 bytes Database Buffers 771751936 bytes Redo Buffers 9269248 bytes Database mounted.
Drop the database
SQL> drop database; Database dropped.
Then you can use dbca to delete the SID and other entries from the server