Unfortunately what you are asking for isn't the easiest thing in game development, so I can't provide an excellent example.
Assuming your sprite is at a starting location, as soon as a new target is established (your touch point), you need to run a calculation to determine the path.
You can store that path as a number of Vector2's (for 2D) and then have the sprite begin to move toward each point in succession until it reaches the end.
Unfortunately there are lots of variables, such as what if something moves into the path you've established? You would have to recalculate the path.
Here's a decent example (See first answer)