If some of the tiles have already come loose, the adhesive used to glue them to the wall was likely inadequate. Try wedging a metal putty knife beneath the tiles to see if you can pop them off, but some of the glue may remain on the drywall, or, more likely, bits of drywall paper and chunks of drywall will pull off with the tile.
If the tile comes off with relative ease, pulling off just a bit of the drywall paper covering or leaving small bits of adhesive on the wall, you can sand down and skim-coat the drywall. Skim-coating involves rolling thinned joint compound over the drywall and pulling a large trowel over the wet compound to create a smooth wall surface. If you pull chunks of gypsum out of the drywall, however, you should replace the old drywall with a new panel.
Because it is usually difficult to remove tile from drywall without damaging the drywall beyond repair, contractors will often cut through the drywall that contains the tile and pull the entire unit -- tile, drywall and all -- away from the studs. This minimizes the mess of trying to remove individual tiles.
Replacing the old drywall with a new piece involves trimming away a section of drywall that extends to the middle of the nearest studs on both sides of the damaged section. Measure and cut a new piece, and attach the new section with drywall screws. Cover the seams with self-adhesive drywall mesh tape, and fill the seams with joint compound. When the mud dries, sand the seams and apply two or more thin coats of mud, sanding between each application.