If you have the system user who installed the database you can enter SQL Plus as DBA user, without entering a password as follows:
1. Enters the system with this user.
2. From the command line, go into SQLPlus typing:
> sqlplus "/as sysdba"
If you need to enter using this way because you forgot the password of a user, you can easily change it:
SQL> alter user user_name identified by new_password;
It can be more than one Database installed on the server, so you have to validate that the environment variables of the Oracle's user are pointing to your database.
For verifying that you has login into the correct database you can execute this statement:
SQL> select name from v$database;