While you can make educated guesses and ask online on message boards, the truth is, your bottleneck may vary between games or applications. For this reason, advice regarding which component to upgrade may not be valid for you. For example, MMORPGs tend to tax the CPU much more heavily than other games.
This guide should prove helpful for determining which component is the bottleneck for each individual game.
How to Know Which Component is Bottlenecking Your Gaming PC | Delidded Tech
If the above guide is too difficult for you, if you name the games you play, your system specs, and your budget for upgrading, I personally can recommend what to upgrade. But I strongly recommend doing the steps in the guide, as they can determine our exact bottleneck for each game.