At spawn turn left and walk (or fly!) straight ahead. To know you are on the right track you will pretty soon have to cross a river to get to a little island. Get on the other side of the island and you will spot a hole in the ground (shown on the third image below).
Make your way down the hole until you see some lava. There are lots of gold and diamonds down there even if we just show a few examples below we can guarrantee there are more.