All the documentation you need to start out if available at :- http://docs.oracle.com/cd/E40248_01/nav/portal_1.htm
As a starting point why don't you have a go at installing and deploying the products you will be administrating, the standard deployment guide should help you there, you will also need to gain product knowledge if you are going being an administrator.
Normally what will I be responsible for? I went throught the standard deployment guide. After the installation and configuration what will be my normal activities in this?
Also we are implementing SOA also, should I consider that in any way?
Also how do I get product knowledge?