Replacing the CPU usually means replacing the motherboard, which can then further cascade into compatibility issues with the type of RAM you have or slots for other hardware. In the end, it's not really something I would encourage a newbie try without supervision.

Anyway, knowing what OS it is running on (Windows 7, 8, 10, etc.) is also an important factor, particularly relative to 32/64-bit. 16gb of RAM is more than likely overkill if you don't do video/streaming or HD art work, with the most likely culprit of bottlenecking being your video card, as mentioned, with perhaps a nod to picking up an SSD to port your OS to and run more vital programs from. Again, though, this requires finding out what slots your motherboard supports (Stuff like PCI, PCI-E, SATA, RAID, etc.) so you get the right parts or don't have to further haggle with adapters if it's even possible to. Further advancements may also mandate a more powerful power supply or PSU and potentially additional cooling so it doesn't all overheat.

If said PC is actually a laptop, then pretty much forget anything we said and know you're more than likely stuck with what you got.