read following:
1) getting started with the database: http://www.oracle.com/technetwork/da...iew/index.html
2) getting started as DBA: http://www.oracle.com/technetwork/dbadev/index.html
3) read and study the documentation http://www.oracle.com/pls/db112/homepage

The last one also has the installation guide.
Since you do not specify which OS you have on your laptop (Linux, Windows or Mac), this is the best we can advise you.
Before starting to be DBA, it is very important you know the database basics (datamodeling, principals of databases, mechanisms of concurrent transactions, data consistency, read consistency etc.)

Good Luck