We need the make and model of your cpu, download and run CPUz and it will tell you the details. Post them here and we can help you choose a upgrade path. Also it would be better if you give us the motherboard information ( can be found in CPUz too)
If you want to upgrade just the CPU, we'll need more information like what exact CPU it is, what motherboard you have, what you use your PC for(games, video editing, office work), and what your budget is. Depending on what processors your motherboard supports, and what your needs are, we can suggest an upgrade then.
If you want to upgrade the PC as a whole the best upgrade path right now is either 8th gen Intel or AMD Ryzen. You will have to change the motherboard for either upgrade, and RAM depends on what your current PC has - if it is DDR4 RAM, you won't need new RAM. For that again, you'll have to tell us the budget and what your PC currently has.