will I have to activate phone first on my mom number the put my sim in or just call AT&T
If you upgraded using AT&T's Next financing plan, you will need to activate the phone with the your mother's sim in the phone first since that is the number it would be tied to when you upgraded. Once that has been done, you can then just swap the sims.
You can also call AT&T first once you have the phone in hand to let you know you wish to activate it on a different number other than the one that was used for the upgrade. They should be able to help you get it activated on your sim directly but it has be done over the phone with AT&T.
I have run into this issue on my account before when upgrading one line and then trying to activate it on one of my other numbers. The phone tells me that it must be activated using the sim and number tied to the upgrade, or I will need to call AT&T to have them activate it on a different number.
This only applies if you purchased the phone as a upgrade with AT&T Next.