Once you have spawned simply dig straight down. After digging for a while you will reach an abandoned mineshaft.
Be quick because otherwise there’s a chance that some lava will set everything on fire. So when you get down there try to locate a source of lava and then block it off.
The ores you will find the most of in the mineshaft are gold and iron. It actually seems to be more gold than iron so that’s definitely a good thing.