Baby Blue Tit

My sister found a baby (what we think is a blue tit) bird on our grass. It is very small, and it was calling for its parents. We left it for a few hours and then came back to check on it and it was still there, so we are getting a bit scared. I then got worried, as it is starting to get cold, so I gave it some hay and twigs to sit in. Should I feed it? Where should I put it?