Adding two lines to form a Family plan would not affect your lines contract at all.
You would either go to an AT&T store to choose your phones or do it online, thru the AT&T site. You choose the type phones you want and any features which are required such as DATA. You can also add which ever messaging plan that will fit your needs.
Depending on which phones you choose, there could also be activation fees for each line.
Each line has it's own 2 year contract.
Some phones are FREE and other's are at a reduced cost. It all depends on which phone you choose.