Breastfeeding & formula?

I'm very weary of what I want my child to consume. I plan on breastfeeding but am so confused about supplementing and using formulas. I don't know if it would be a good idea to. I got a couple of containers of powdered similac. Should I just pass these on to someone who will use them or should I keep them in case? I also know that powdered formulas are not sterile. Any input is appreciated.