Regardless if you have the best gear the game has to offer it is not going to be much use to you if you can't take down your targets with relative ease. Unlike similar games in the genre there is no type of tutorial mode that enables you to practise with the different weapons to see how each of them handles. In PlayerUnknown you are literally thrown into a live battle so there is no opportunity to practice shooting at the enemy in different scenarios without being quickly taken out.
One way which you can practice your aim is to play the Squad matches as a solo player and get the experience that way. Below is a diagram indicating where you should be aiming for at a certain distance and two great videos that explain in detail the different factors you have to take into consideration when aiming at a target. This includes how to utilise the scope when shooting enemies at different ranges and how to hit moving targets.
100m - Tip of the chevron.
200m - Bottom of the chevron.
300m - Top of the line.
400m, 500m, 600m - First notch, second notch and third notch on the line respectively.