For the t-shirt you want to stencil, i'd suggest if you're a newbie sewer, to go to wal mart or a thrift store, buy a plain tee for $4 and stencil on that.
My thoughts are, if you are just learning to sew, you may want to build your confidence sewing some smaller, easier things. But it's all up to you. I think sewing a whole tshirt might be a big overwhelming if you're not used to the sewing machine, and so on.
And in terms of the plain white tee u want to recon, why don't you cut off the collar and stencil on that? Or else, turn the shirt part into a halter, and the sleeves into headbands?
Just some suggestions, hope that helps!