I did have a couple nursing shirs and they were great for those first few times out nursing in public. I however had two nursing cami's, and a couple nursing tank and those things were GREAT! you could wear them under any of your regular shirts and viola, a nursing shirt. :) Also I bought some of the super thin Hanes undershirts, and cut slits in the sides to the outsides of my breasts/under the armpits, so I could wear those under any shirt too and then just unhook my bra, slide the slit over, and then lift my top shirt up so my belly stayed covered.
Also Old Navy sells really thin tank tops, so you can lift your top shirt up, then pull the tank down from the neck to nurse.
I always wore an undershirt, tank, or nursing cami/tank under my regular clothes, mostly to keep m ybelly covered while nurisng in public. :)
Congratulations!