Oracle Database Discussions

I have an oracle database (or instance) named viton1 on my linux.
Now I want to create another instance (or database) named viton2 on my linux without using dbca.
I know I can create a database by runing a sql script : create database ......
But my problem is:
before I can run the script for create database, I must startup nomount database.
Before I can startup nomount the instance, I have to connect the instance.
So I do it:
$sqlplus /nolog
sql > conn sys/password as sysdba;
sql >
then I connected to viton1, that's not what I expected.
In order not to connect to viton1, I set ORACLE_SID=viton2. Then it show error: insufficient privilege.
Who can tell me the right way to create another database without using dbca?
Thanks in advance.