Rather than reinventing the wheel, I am pointing the way to a piece by Denny Rickards. It is actually his site promoting the sale of various fly lines. In the promotion of the varied lines is a lot of good information that will benefit the stillwater fly fisher that has not quite sorted out the presentation part yet and still only carries a floating line. You would be well served to primarily use a ‘clear’ Intermediate as your primary line and from there consider the floater and perhaps a faster sinking line (less used). I do not promote any sales on this blog. I just believe Rickards does present excellent information about presentation, which is often as important or more important than fly selection. Rickards also had several informative books out on the market that worth a read if you are learning the basics of stillwater fly fishing.