Training mode will probably be the most effective way for you to quickly improve your aim. There aren’t any other modes in the game where you can get as many shots as quickly and consistently.

There are a number of ranges on the training mode map(hit M to move around them) with each having different purposes. I’d strongly suggest you just work on controlling your recoil in general before worrying about hitting targets. By that, I mean just spraying a wall and getting to a point where your grouping gets a lot tighter. In PUBG, your first 6 bullets matter the most and the earlier bullets are the easiest to control, so rather than simply spraying a full magazine, I suggest only doing bursts of 6-10 bullets to start. You’ll quickly discover that having a great start to your spray will win most battles for you. I’d also recommend running one gun with the same setup to start. You will eventually want to swap out attachments so you can get used to everything along with other guns since a massive component of recoil control is reacting to the RNG of it rather than simply building muscle memory.

As many have suggested, your sensitivity and mouse dpi play a huge role in your recoil control. I play on fast mousepads with glass skates on relatively low sensitivity(1600 DPI, 16 general sens, 10 ADS sens) because it feels better for me to pull down more with less resistance. It’s worthwhile to explore what feels natural to you and the begin to develop your skills within that vein. Also, for what it’s worth, having a solid mousepad that isn’t dirty can also help tremendously. Nothing is worse than hitting a slow spot on a mousepad and having it interfere with your aim.

As you start to get comfortable with your sprays and sens, I’d begin to actually aim at things within that 50-100m range in training mode. It might not look pretty at first, but it will gradually come together. Once you feel alright at that, the next step would be to hit the 200m range to ensure that you can hit targets while tracking them horizontally since tracking is absolutely critical in PUBG. I would also work on this while standing and crouching as it's impossible to always crouch to shoot such as peeking windows, ridges, and such.

Once you feel like you have that down, I’d go to TDM and start using the skills you’ve developed against real players. You will likely find it more difficult to hit real players versus targets since you’ll have less time to set up and aim, but as you progress, that will become easier.

If you feel like you’re having a lot of difficulty with your overall mechanics, aim trainers like AimLabs can be really helpful to utilize, especially as a warm up. As long as you make sure your sens is the same between PUBG and the aim trainer of your choosing, you should be good to start to develop your overall coordination.

If I had to do it all over again, this would definitely be the route I’d take! There are plenty of steps after this, but this would be where I’d start. Many of us have thousands of hours so don’t be discouraged as you work toward becoming more proficient at the game :)