Alot of people recommend normal 'Argos' play-sand as it's cheap and does the job. I bought proper tank sand from my local M.A, £15 a bag! Anyway, measure then scoop out the amount you are going to use in the tank (add a little more) then clean it out the back..maybe put it in a CLEAN bucket, put a hosepipe into it and let the water run out at an angle for an hour or so, maybe more, depending on how clean it is/was when you bought it.
I cleaned mine for about 2 hours but my tank still went cloudy for 2/3 days.
Regards,
Terry.