Remove ORA-01100:DATABASE ALREADY MOUNTED
You seem to mixup the terminology for database and schema in an Oracle database. In Oracle up to 11g, a database is just a collection of files. Those files are logfiles, controlfiles, datafiles and tempfiles. Tablespaces are created using datafiles and you create tables and indexes inside a tablespace.

If you want to create tables, or other schema objects, you create them with a chosen user as their owner. Such an owner is a schema and there can exist multiple schemas in one database. Schemas can share tablespaces, if you want to. Often, tablespaces are created dedicated for a schema, giving a bit more oversight in who is using what resources.

Since Oracle 12c there also is the option to create a pluggable database inside a container database. Once that pluggable database is created, you can connect to it and use it in a similar way as the pre-12c database, where you again start creating users/owners/schemas and their objects.