Start by "understanding" your requirements. What do you want your PC to be capable of doing with respect to performance, storage, graphics, etc.?
Then look at your motherboard. Review its' specifications and requirements.
Then look at whatever new component you are interested in. Ensure that it matches the requirements (minimum, maximum, recommended) imposed by the motherboard.
Your end system will likely have its performance dictated by the slowest component.
For example you can often install "faster" memory but that memory will actually perform slower based on the motherboard's specifications.
Your mother board may or may not be able to accept a faster CPU.
The motherboard may or may not support a higher end GPU. There may not even be a slot available.
Boils down to matching the specifications. If you can identify a component within your system that is "slower" than what the motherboard can support, then that component may be upgradeable to a faster, higher performing version.
Know your motherboard and what component specifications it will support.
E.g., Very common that a motherboard will have some memory limit. Exceeding that memory limit will do no good at all.