Installing scott schema for oracle 11g express
when you are installing oracle database you can install all oracle example schemas like scott, HR, oe ... 
you can install these in step "Specify Database Config Details".click the last tab page "Sample Schemas" then check the check box "create database with sample schemas" as displayed bellow in screenshot.

if you already installed database you can recreate it for training "without uninstalling it" by write the command:
start -> run -> dbca
to run oracle database configuration assistant.
follow steps to drop your running database and recreate it with sample schemas.



update

and here in Oracle 12C: