Adivce - Prep for painting outside of house.

I did do a search on advice and all I can find is people saying to make sure you do plenty of prep work first. But what prep do I need to do...

It's a smooth plaster and it was plastered about 2 years ago.
There are a few hairline cracks in the plaster, do we fill these and with what?

Can anyone give me any advice on the prep, I'm getting mixed advice from the guys in the paint shops.

Thanks,
Tilly.