In the movie "Searching for Bobby Fischer," there's a famous quote, "Don't move until you see it." It comes from a coach who is telling his chess-playing star not to move until he sees the line of moves he's about to play. Similarly, in table tennis (which we often call "Chess at light speed"), you shouldn't play a point until you have a game plan.
If you don't know the opponent, then the game plan at the start is a mix of forcing your game on him, and figuring out his game. Sometime in that first game you should have a basic game plan worked out. The game plan should be flexible, subject to change as needed. Against an opponent who gets used to or adjusts to what you do, you have to make changes. It is likely different when you are serving than when you are receiving. And the plan changes as the rally proceeds. But at any given time, you should have a basic game plan.
Here's an example of a game plan, one that I might use. On my serve, my basic game plan might be to serve a lot of short backspin, hoping to get a long push that I can loop. If my opponent pushes my serves long, and I am winning by looping those pushes, then my game plan is working. But suppose the opponent's pushes are very good and I have trouble with them, or he starts flipping or pushing my serve back short, and I'm unable to loop. Then I would have to change my game plan. For example, if he's pushing heavy or dropping my serve short, I might serve more no-spin serves, which are harder to load up with backspin or to drop short effectively. If he's flipping the serve, then I'd focus on varying the spin as well as making sure I'm serving very low. (In reality, my game plan would more likely have been to mix in short backspin and no-spin serves, so the opponent has to adjust to both - and I'd soon figure out which one is more effective against this opponent, and favor that one. Plus, I'd mix in long serves so he can't lean over the table waiting for the short serve.)
On the receive, my basic game plan might be to attack deep serves, so the server would be forced to serve short. Against short serves, my game plan might be to go for a consistent flip into the backhand, trying to force a neutral backhand exchange and thereby taking away his serve advantage. But suppose I have trouble flipping his heavy backspin serves, or perhaps he has a very good backhand and so this tactic doesn't work, and if I flip to the forehand, he attacks even better. Then I might decide to instead push his short backspin serves aggressively to his backhand, and see if that works. If he's able to attack them, then I might try dropping them short and low, making it harder for him to attack. Eventually I'll work out which of these is most successful, and favor that receive, though I'd still vary it so he can't get used to any one thing.
What's your game plan? Don't play a point until you have it.