Best to skim coat over your existing texture then apply a new texture if desired.  To do this start by applying a good acrylic or latex primer, allow to dry, then trowel on 1 or more coats of all purpose drywall mud.  Allow the mud to dry then sand smooth.  Much easier than trying to sand off hard paint and texture.  If you still want to sand off your texture use a palm sander with rough sandpaper, 30 grit to start.