The clay bar removes contamination & overspray & such, basically pulls that junk out of the paint surface. Use with water or better yet, spray detailer. Do it after washing the car. After the clay bar you can compound, polish or wax depending on what may be needed.
Life expectancy of wax depends on the product and the environment the car is subjected to; garage queens can go years, a black car that's kept outside is more like monthly.