You will need some pebbles which are smooth, but not totally shiny (the paint won’t permanently stick to waxed garden pebbles or marble). Where I live there are many suitable pebbles in the stream.
Inspiration is easy to find. Just Google “rock painting easy” and look at the images tab. Simple and bold designs e.g. emojis, hearts and rainbows are awesome.
You need acrylic paint or metallic markers.
I don’t recommend Sharpies, as they can run when you apply varnish (our youngest drew a pretty little girl with Sharpies, which to her horror morphed into a scary clown when varnished).
It is pretty fiddly to mess with paints and paint brushes, so I almost always use pens.
Metallic markers are cheap and easy to use, like these ones: https://www.amazon.co.uk/gp/product/B07LGSDJQX/ref=ppx_yo_dt_b_asin_image_o00_s00?ie=UTF8&psc=1
Acrylic paint pens, preferably POSCA but cheaper ones are available: Posca PC-1M Paint Pens or for slightly thicker ones PC-3M
I always use this pen to do outlines, which makes the whole thing look neat and tidy: Posca PC-1M Paint Art Marker black
I have this set, which is great as it has both the bright colours and the black for outlines: https://www.ebay.co.uk/i/283196764159
One the paint is dry, you will need to seal it with varnish. I apply yacht varnish with a paint brush, but it is far easier to use a craft sealant like this: Plasti-kote 1138 400ml Super Clear.
Spray varnish fumes are toxic and can get on your chest, so this should be sprayed outside or in the garage.
I always put my pebbles on an old cake cooling rack while I varnish them, with a piece of newspaper underneath so our garden bench doesn’t get all shiny.
You can use the painted rocks to decorate your garden or your hearth, or hide them around your local parks so people smile when they find them.