What step should I take to move my 15 year lawsuit along?

There is no set answer to this question because it is very fact dependent. On the other hand, there is no reason I can think of why ANY case should take 15 years. In fact, most courts would send a notice before then that they are discontinuing the case for "Non Pros" (in other words, because neither side is doing anything).
Assuming the case is still active in the court, there are any number of things that could be filed to move it along, depending upon what its status is right now. Have both sides answered all pleadings? Have you gone through discovery (interrogatories, depositions, etc.)? Have any motions been filed? Sometimes local courts will allow cases to sit indefinitely if neither side files a Precipae for Trial. I know that sound ridiculous, but it's true in some Pennsylvania counties.
If your lawyer isn't moving the case along, and it is still active, perhaps you should consider getting another lawyer who will be more aggressive in protecting your interests.