To remove polystyrene ceiling tiles, make sure you have a stepladder tall enough to enable you to reach the ceiling without overstretching, as this is dangerous.
The ladder should be on a flat, stable surface and your floor covering should be protected from falling debris, unless you're planning to replace it.
You'll need a wallpaper-stripping knife to remove the polystyrene tiles, but be careful not to damage the ceiling underneath, especially if the knife's blade is sharp, because plaster can be soft and prone to gouges.
Once you've removed the tiles, you're left with the problem of the adhesive they were fixed with - use a hot-air gun to soften it before scraping it off. Try scrubbing off any remaining adhesive with an abrasive sponge dipped in solvent thinner.
Be careful when using the hot air gun, as, again, you don't want to damage the ceiling - or yourself. Play safe by wearing protective gloves and goggles, and remember to ventilate the room.
It's also a good idea to wear a shower cap to protect your hair from the debris. If you're struggling to remove the adhesive or the ceiling's damaged, you'll need to skim it or get a plasterer to do it.