Nepeta is native to dry areas, so it likes soil that is not too wet. In the garden, you rarely need to water it. If you plant it in pots, be sure to allow the soil to dry well between waterings. It may be helpful to use hydro pellets in the bottom of the pot, and mix a little sand through the potting soil to keep the soil nice and airy.
Prune the plant back to about 10 cm above the ground before the longest day (around June 21): this will guarantee the Catnip a second bloom!
General care tips can be found here