Supplementing with formula?

I'm a ftm, and I just want to know if anyone has any experience with supplementing with formula while breastfeeding. My daughter is 3 months old and has been exclusively breastfed since birth. But I feel like I have a low supply and sometimes I feel as if shes not satisfied after shes done feeding. I was hoping to switch one feeding time a day or a few times a week so I can pump and start a supply. I have nothing in the fridge or freezer at this point because after every feed, shes normally hungry an hour or two later so I dont get the chance to pump. But, can I start right away or is it too early? Im not too sure and I'm a little nervous about giving her formula. I somewhat feel like a "bad" mom because I feel like I'm "giving up" with breastfeeding. I just dont know how it'll affect or change her? Will she prefer formula? Will she be more fussy at my breast? How much formula should I give her? Not sure. Please help!