When you built your database, you should have been prompted for a password at some point for the system and sys accounts. That's the password you need to use.
The comment above about logging in with "as sysdba" with the user SYSTEM is incorrect. SYSTEM does not have SYSDBA privilege.
If you've forgotten or don't know the password(s) you specified for these accounts, you can sneak in through the back door by doing the following (assuming Windows):
- Add your Windows user ID to the ORA_DBA group.
- In a command window, set ORACLE_SID appropriately and enter "sqlplus / as sysdba"
- You should be logged in as SYSDBA. You can now reset the passwords as @Gaius stated.
- One other potential gotcha is that you may need to modify the sqlnet.ora file in your ORACLE_HOME/network/admin folder to contain the line SQLNET.AUTHENTICATION_SERVICES=(NTS) if you continue to get the "insufficient privileges" error in step 2.