Adding a new line with a new phone help

I want to buy a new phone (first time). Also I want it to be a additional line meaning it will be add on a family plan.

First, should I buy a phone directly from a Tmobile store, Tmobile online, or other online store (ebay, etc.) ?
What do I need to do so it will be add on a family plan?
Will there be any complication on doing this?

I don't know much on this especially my mom.

Phones I am thinking about to buy:
Nokia 3660
Samsung E315
Samsung E715
Motorola V600

Should be no problem at all, go to the t-mobile store get a phone and whatever plan you want and let them know it going to be added on another line (they should give you a blank sim card) go home call t-mobile and they'll step you through the rest.

Sounds good to me thanks for help