How to swim better
The first thing I would say is to not practice anything wrong. If you have a coach, then definitely ask him/her for advice. As far as I have seen, the most challenging and important things swimmers struggle with is flip turns, starts, breathing, and underwaters.
Underwaters (as in swimming underwater on first dive in and after flip turns): Make sure your hands are completely straight tight up to your ears, and that your legs are straight while doing dolphin kicking (depending on what stroke you are doing). Also make sure your head is not looking forward, but at the bottom of the pool. This and the straight hands allows for more aerodynamics.
Breathing: One of the most important things about swimming is breathing. When underwater, you should be blowing all of your air out so that when you come up, you can get more oxygen. Instead of wasting time breathing out, you just get to breathe in.
Starts: When starting off a race for the freestyle, butterfly, or breast stroke, always have your dominant foot out in front. Your toes should be slightly off of the blocks for more grip (only with your dominant foot). The back foot should be positioned where comfortable, but about halfway or three fourths of the way across the block (ask coach or someone who knows you for more info. Just depends on the length of your legs.) Another important thing is that your head is looking back. What I mean by this is that you are almost looking through your legs, which might feel strange, but it prevents your googles from falling off when you hit the water. When you start, push off as much as possible to get the fastest speed into the water. You shouldn't "fall in" but you shouldn't fly in the air in a verticle direction. Somewhere in between to be going the fastest.
Flip turns: Flip turns can slow swimmers down a lot, of it can speed them up. It is important that you have the form right. You should never breathe past the flags because that also slows you down. It might be hard at first, but practicing it this way is important. When doing the flip turn, try to flip straight without turning. When you hit the wall, you should feel like you are sitting in a chair, or at a similar angle. Make sure you are also putting full force into the wall when you kick off. You should begin the underwater facing the sky. After you kick off, you should turn, not before. Try to not come up until after the flags. Also your first two strokes you shouldn't breathe on because it will slow you down. Try taking a breath on the third stroke. It will be hard to hold your breath for tgis long at first, but it will make you faster.
Hope this helps:)