If you are agile, you want the flexibility to change. Agile is all about change. My question to you is why do you want to plan for a year? Did you read Agile 101, with the link to the manifesto? You should read the principles behind the manifesto, too.
If you plan for an entire year, you will be disappointed. There is no way the teams will deliver what you want. No way. Why? Because they can't read your mind. Because the world will change. Because you need to respond to change.
I suggest you read my book on project portfolio management: Manage Your Project Portfolio: Increase Your Capacity and Finish More Projects.
You are thinking waterfall (no surprise, since you are new to agile). Instead, think about flowing work through teams, as you need projects (or work) done. You will be happily surprised. It's a challenge, and it works.
Good luck. Oh, and when you get through this year, write me an article, ok? I'm the technical editor for agileconnection, and I would love an article from you!