Clay wash is clay that's heavily diluted, with added pigment to produce it in different colours. There's nothing else in it, so it's pretty much non-toxic. the only thing I add to it is a tiny drop of washing up liquid that reduces the surface tension and helps it flow over a flat surface. I use Flory Wash, it's not widely stocked in model shops, but you can get it direct from the Flory web pages. Phil Flory does demo videos that you can find on Youtube, for instance, here. If you're going to use clay washes it's important to note it must be used over smooth/glossy surfaces, overwithise it's difficult to remove and will stain the paintwork.
I'd recommend having a look at the tutorial by Plasmo if you want to try oil paint washes, you can find it here.
The Humbrol tutorials are also worth checking out for 'how to' use Humbrol enamel wash.