10 to 15 feet is deeper than most fly fishing setups will effectively reach. It is difficult to fish that deep with a floating line and poly leader.
You can get to 10 feet or so with a long-ish headed sink tip line. The head needs to be longer than the depth you are trying to fish. The sink rate of the line depends on how fast the water is moving. In fast moving water, you'll need a fast sink rate to get to the strike zone before the line is moved down stream.
To get to 15 feet, I'd probably either get a sinking head setup or a full sinking line. Where are you planning to fish? I personally don't like using full sink lines or heads in rivers, because I tangle them up too often.
Fly Fishers International certified casting instructor