You can use Third Party Tools like Toad or Navicat which have excellent GUI to get you logged into Oracle

But if you want to go the traditional way; find below instructions

• Open Command Prompt
• Type:  sqlplus sys/password@<hostname>/<global service name> as sysdba
• Typing the password in this manner is never recommended; instead use below option
• Type: sqlplus sys@”<hostname>/<global service name>” as sysdba
• It will then prompt to enter password
• An alternate way is to log into SQL initially without a user
• sqlplus / nolog
• Now under SQL Prompt; type connect sys@”<hostname>/<global service name>” as sysdba