Don't touch your AT&T line. The act of porting will automatically cancel your old line.
So 1) Buy your new phone
2) Activate and port in your old number
3) You'll get an AT&T bill for your last month + Early Cancellation Fee.
You will need the account number (and passcode, if there is one) from your AT&T account. You can get this from a recent bill or by calling AT&T customer service.
Do some research on Net10- you have the choice between Net10 on the AT&T network, on the T-Mobile network or on the Verizon network, and you can choose by closely examining the phone info before you buy (either the website URL or the physical box). The EM326 is either a T-Mobile or AT&T network phone, depending on which SIM card is included.