I'll try to help, brief at first. I've been shooting a Glock 17 or 34 in competition for about 8 years. They are one of the most accurate pistols I own. Every new shooter puts shots low with a Glock.
1. Slow down, focus on each shot.
2. Get a smaller target. Multiple 1-2" dots at 7 yards is a great place to start.
3. Learn the dime drill. Balance a dime on the front sight of your UNLOADED pistol. Present it, get a sight picture and dryfire without dropping the dime. Repeat.
4. Stage the trigger when firing. Pull the slack out of it, feel the tension increase and then slowly add pressure. Wait for the break. Do not think "FIRE!" in your head, think "Wait for it" "Squeeze" or anything that works. The commanding thoughts like "fire" or "now" make jerked shots.
5. Keep the trigger to the rear after your shot. Release until you feel the connector and triggerbar reset. Now re-apply pressure and shoot again. This is "Shooting from the reset", handy for many guns, essential for the Glock.
6. If at any time you slip in live fire, stop right there and dry fire. The fundamental I tell any new shooter is to pull the trigger without moving the sights. Somehow that often gets left out as the overarching goal of all this sight picture, sight alignment, trigger control talk.
7. See your sights when you fire. With time, you will see everything they do. If you didn't see a muzzle flash, you closed your eyes. Flinch happens, dryfire it out of you.
8. Glock stock sights suck, but I digress. Anyway they are registered so that POI is about where the front dot in the sight is if you align the dots. So on a 1-2" circle at 7 yards, a center hold will probably put you center target.
All this boils down to don't jerk the trigger you trigger jerking jerk you!
Well gee, that's simple, but we all find ourselves there at some point.
Perfect practice makes perfect, slow down and make them perfect. Good luck to you, there are simple mods that can make it a lot more shootable too, but box stock will do fine if you will.