Pump it out for a while. If the pump controller does not have a "low pressure cutoff" pay attention so you don't leave the pump on if you manage to pump it dry.
Common to "shock treat" (chorinate, recirculate, let sit, then flush out) on general principles. If the cap has been in place these 15 years there really isn't anything that should have gotten in, but general principles...
Depending what you are doing (such as drinking the water) might also be advisable to test the water (generally after flushing out quite thorughly and over several days, if you did shock-treat it), to be sure there are no issues with water safety from bacteria or other contaminants.
If your state (or other local authority) has a registration program you may be able to find data about your well from them (as well as, or instead of your well driller) - check to see if there's a plate or something with a registration number for the well on it.