MLB the Show is a very simulation oriented baseball game, so you need to play as if you are actually playing real baseball.
In particular, your issue is that you haven't developed good plate discipline. You must develop a good strike zone judgement to succeed in this game. This one of the most frequent issues about MLB the Show, that hitting is too hard for gamers. In fact, hitting in baseball is one of more difficult skills in sports, so try to enjoy the process of improving your plate discipline!
One thing that distinguishes MLB the Show from other video baseball games is that pitchers do frequently throw pitches outside the strike zone. In other video games, pitchers tend to throw mostly strikes, which you can at least put in play. Not in MLB the Show. In fact, if you keep swinging balls, the game AI is smart enough to pick up that tendency, and starts throwing a lot of pitches outside the zone, to make you chase.
So how do you go about improving your plate discipline? My favorite approach is to start at the Rookie or Veteran hitting difficulty level. At those levels, AI pitchers tend to start an at-bat with a deep strike in early counts. Those are fat fastballs around down the middle of the strike zone, which you should try to put in play. If you see something else, just lay off, even if you think it is going to be a strike. Just concentrate on hitting fastballs down the middle.
Repeating this many times, you will start recognizing the pitch trajectory for fat pitches down the middle. At the same time, you will develop discipline to lay off everything that's not down the middle. Once you master this, you should be able to hit over .300 at those levels. If you are comfortable, then increase the difficulty level to All-star. There, AI pitchers won't start off an at-bat with deep strikes, but that's where things become fun.
So if you are a beginner, the key is to swing only at pitches down the middle. Try not to do too much unless you become comfortable with this.