To install the SoA schema which is called Repository Creation Utility (RCU), I will assume that you already have Oracle databse 10g Express installed in your machine, if not, you can follow the instructions given here to install the database.
Then, we need to change some database's specifications to satisfy SoA's requirements.
Firstly, the Maximum System Global Area (SGA) Size of the Database should be more than 150M,
To do so, go to your databse at http://localhost:8080/apex,
then Administration------>Memory