You want to set up a new line/phone number for your first generation iPhone under an AT&T Family plan or as a separate individual plan?
You can't activate your first generation iPhone under your existing line/number for your iPhone 3G, which was transferred from your first generation iPhone, and share the single line and data plan for your iPhone 3G between both iPhones. If you reactivate your first generation iPhone with your existing line/number being used by your iPhone 3G, your iPhone 3G will no longer be activated.
If you want to set up a Family Plan which will have shared minutes between the two iPhones with your first generation having a different line/number, you may need to go back to your local AT&T store to get this started. The additional line for your first generation iPhone will cost you $9.95/mo for the line, and the data plan for the first generation iPhone costs $20/mo and includes 200 SMS.