Fundamental tactics are the standards used against specific playing styles. If you are playing a chopper, fundamental tactics include attacking the middle (roughly the elbow, the transition point between his forehand and backhand chops), moving him in and out, giving dead balls to a side with long pips, or mixing up your spins (especially to the inverted chopping side). If you are playing a looper, a fundamental tactic would be to serve short and return short serves back short, so he can't loop, or to go to his stronger looping side to bring him out of position so you can then go to his weaker side.
You can learn fundamental tactics talking to other players or coaches; watching others; experimenting; or even by reading about it.
While fundamental tactics are key, it's also helpful to learn to be creative. Everyone plays different, and everyone has different weaknesses. That means studying opponents and finding what specifically gives them trouble.
Perhaps your opponent has a less common grip - say, the Seemiller grip, where he uses one side for both forehand and backhand. If you attack the middle like you should against most shakehand players, you might not do so well, since the Seemiller grip is very strong against these middle shots. But it's usually not as good against shots to the wide corners, especially the wide forehand. They often have trouble with the wide backhand as well, but make up for this by standing well over to that side, turning that into a strength, but leaving the forehand a little more open. But most players just automatically play to their backhand. So watch how this player stands, and go after those corners - perhaps going to the wide forehand first, then back to the backhand, which is now open. (This grip also has trouble backhand looping, so you might take advantage of that as well.)
Or perhaps he puts his finger down the middle. Some players with this grip have trouble returning short balls to the forehand down the line - he'll likely be forced to go crosscourt, since it's often harder to bring the wrist back with this grip. So you test it - serve short to the forehand, and if it works, you can camp out on the forehand side for the return. (Some have no trouble going down the line, so test it out.)
Or suppose you're playing a chopper who returns your attack to his middle with ease. Then he's probably cheating over - meaning his middle is more toward his forehand side, and his wide backhand is open. So you punish him with those two spots, which are now far apart and hard to cover.
Or suppose he's a looper, and so you don't want to serve long to him, giving him the loop. But test him on this. If he's like me, then he might loop some serves really well, but struggle with others. (I always had trouble looping deep serves with backhand-serve type sidespin.)
The goal is to early in a match find out what fundamental and creative tactics work. If you combine these two, you'll become a master tactician!