We could probably write a few dozen paragraphs about what to do with the first ball in the form of either a first volley or half-volley, depending on lots of things. Where are you in the court, is the return down-the-line, cross-court, high or low over the net, where did your serve put the returner (wide, toward the center, neither), what's the returner's weaker side... just the tip of the ol' iceberg of variables.
If you can put your opponent on the run, that can be a strong option, but if your first shot can simply go to that opponent's weaker side where he/she can't hurt you, that's great. You're going to find that as you develop S&V instincts, you're quite basically looking to keep your opponent defensive as you gain a better position.
Whenever your first shot can be placed deep in your opponent's end, that's just about always the best option. It keeps them out of the driver's seat when they're pushed back from their own baseline while you advance. That can mean that a deep shot to the weak backhand side might be better than making a player run to their forehand side if that player has a killer running forehand.
Can you swing your opponent off the court with a wide serve? If so, your second shot should usually go through (or at least into) that open space you've created for yourself with that wide serve. That's the serve and volleyer's "classic combo" I think.
If a tough returner knows how to put the ball down on your feet, that can kill a lot of your initiative as a S&V'er. Work on a strong half-volley so that you can counter this sort of return and retain control of the action more often.