BABY ROBINS EGG FOUND HELP!!?

Ok I found a baby robins egg outside in my garden. I know what your gonna say "OH DONT TOUCH IT THE MOM WONT COME BACK!" And I know that because im not stupid. At first I thought it was just a shell because it was half way under the mulch and about a foot away from the nest, which had fallen out of a tree about 4 feet high. Well I picked it up to look at it because I wanted to show my friend and I realized HEY LOOK ITS WHOLE and then I was like **** beacuse my friend told me that thing up there ^^. I need to know what to do to keep it alive. There where no others and right now its in a brown bag, in the nest with a small stuffed animal over it to keep it warm. What do I do when I have no incubator and I NEED to keep it alive.