I get a message saying "Service Battery", but it continues working ok. How do I service the battery?
The message that you are getting is because your battery is beginning to fail. I can't tell you exactly when, but more than likely, when you need it most, it will no longer hold a charge.
It's actually quite easy to service the battery. There are 10 phillips screws on the bottom. Just remove those (pay attention to where they came out), then you will have access to the battery, memory, and hard drive.
I answered a very similar question on a 2009 MBP battery replacment.
Have a look What's the easiest/cheapest way to replace my battery? 2009 MBP for further details.
I actually have a 2009 MacBook Pro and I took out almost every component of it to replace the keyboard and put it back together again (bought it broken for $20, spent $23 on a new keyboard, and now I have a $600 computer). Anyways, a battery is the first thing to come out, and thus the repair is actually very simple, so long as you have the right tools.
You need to remove the Phillips screws on the bottom, disconnect the battery from the logic board, then remove the Torx screws (I think) holding the battery, and remove the battery.
See this iFixit guide for more info.