Welcome to the BlackBerry world. I'm no expert but here's my feelings on this.
I'm pretty sure you can just go into a Nextel store and have them activate the BB. One thing you have to make sure of when you buy a used BB is if it was on a BlackBerry Enterprise Server (BES). If it was you may not be able to use it or use some of the features until it's released from the BES.
I remember Juno. You will have to see if Juno allows for POP mail access then you can set up your BB to check that POP address. If not then you might be able to use web mail if Juno supports that.
Sprint 8130 Pearl 2