First, try to scrape out all the hard grout from the perimeter of the tiles you want to remove. You'll want to try to isolate the tiles from each other as much as possible.Gently but forcefully push a thin putty knife 2"-3" wide between the first tile you want to remove and its longtime neighbors, moving around the edge and breaking any leftover grout sealing them together. Pry them away from the wall, one at a time. Work around the tile, nudging and prying upwards to gradually separate it from the wall.
Depending on the wall, and what is around it, an old-fashioned hammer can do the trick too!
This depends on what the tile is set on. In a shower or tub enclosure If it is green board or drywall. My prefered route is to remove it wall board and all. 95 percent of the time the wall board is too damaged to be reused. An the new tile should be set on backer board anyway...so stripping to studs is often the fastest and best.