2003 Chevy Impala Engine oil and water mix

I went into a service station to get an oil change because my car flashed the "Low Oil Pressure" light. When I pulled into the service station the attendant told me my engine oil had water in it and that he would not be able to give me an oil change, what are the quickest and cheapest ways to extend the life of my car without putting too much in to save such a "well used" vehicle? Also, since I do not know what the problem is what should the first tests I should try so that I can figure out exactly what the problem is?