As others have said, the first thing to do is take it to the vet and have it tested for any illnesses. Once it's a little older, it will probably need to be de-wormed and treated for common parasites like coccidia (it's rare for kittens born outside not to have these small problems).
It sounds like you already have the kitten formula, which is great, but you need to feed it very frequently (every few hours) at this young of an age (just like a human baby). You'll should only have to do this frequent of feeding for a few more weeks (2-3 weeks max) before you can introduce it to regular kitten food and let it self-feed when you are not home. Until then, do you know anyone who could feed it when you are not around? Or could you ask the vet if there is a way to set up a self-feeder with formula in it?
Please do keep us updated! If you have been wanting a cat, then I say the arrival of this kitten is telling you just one thing: the time has come!