If you logged in to the server as a SYSDBA, you start the default instance with the "startup" command. Here is how to start the default instance in SQL*Plus in SYSDBA mode:

>.insqlplus

Enter user-name: SYSTEM/globalguideline AS SYSDBA

Connected to an idle instance

SQL> show instance

instance "local"

SQL> startup

ORA-00821: Specified value of sga_target 16M is too small,

needs to be at least 20M

Now the server is telling you more details about the memory problem on your default instance: your SGA setting of 16MB is too small. It must be increased to at least 20MB.