Yes, BB usage is an extra fee, with T-Mobile it is $19/month for BB unlimited usage, which also includes some phone time. Depending on your phone usage, it may not be much more than you are currently using.
Once you have a BB plan, T-Mobile will give you a
email addy and set you up. I am not sure exactly how their BIS webclient works, but you generally go to the site, maybe enter the PIN & EMEI from the BB device you have and from there send the "service books", after that you can send and receive emails.
Then I would recommend going into your outlook on your PC or whatever email you use, update your contacts with phone number/email addresses and then sync your BB with Outlook via the Desktop Manager which should have come on a CD with the phone.
You can find allot of information here with a few searches, or just by reading the Sticky threads that have a huge amount of info.
PS. Service books are merely how the BB network/Service Provider sends information to your BB so it knows who it is and what software is on it, sort of configuration data. You really don't need to know about them, other than how to send them every once in a while if your BB stops responding.
The lower case signal, just means you don't have a data connect. Sometimes you just loose it, and you have to power off and on or remove the battery for a minute to get it to come to life.