The earliest multicellular life-form that we know about is probably Grypania. This is the oldest fossilised multi-celluar life that we have found. It is a bit like algae, and lived about 2.1 billion years ago, a long time before most other multicellular life evolved. There were probably some multicelluar life before Grypania, but we don’t know about it because it would have had soft tissue which doesn’t fossilise very well.
Scientists believe that the first kind of life on earth were single-celled prokaryotes – which are organisms which have no nucleus, such as bacteria. The oldest known fossilized prokaryotes are around 3.5 billion years old – which just goes to show the enormous time scale over which life has developed.
In terms of the first multi-cellular organism, a lot of what we know is speculation, because evidence from this time is scarce. We think that the first multicellular life arose some 650-800 million years ago. Some think that multicellularity first occurred as a form of symbiosis between single celled organisms, and some think it formed from colonies of single celled life, like sponges. There are many other hypotheses: