How do I prepare and paint an old, stained concrete driveway?

Prepare it for what? stained with what?

Have you tried power washing it? You can rent a power washer. Just remember, once you paint concrete, you will always have to repaint it as it is porous and very hard to remove old flaking paint and keeping it looking good. If you really want to do this, google how to paint concrete. There are many DIY directions.

https://www.bunnings.com.au/diy-advice/paint/how-to-paint/how-to-paint-a-concrete-driveway

I think pressure washing would give the best results. Have you considered staining it, not painting? It would cover some stains (or make them look intentional), and you wouldn't be locked into repainting it for years to come...

I think this will help https://www.youtube.com/watch?v=yUgKhUcMvDc

Here are some good ideas of what to expect:

http://farmfreshvintagefinds.com/barn-makeover-stain-concrete-part-2/