Need Help Choosing Parts For Upgrade

ltt10

So about a year ago I bought a pc off of my cousin, this pc is about five years old now. Currently has a 3060 12gb, a r 5 3600, 500 watt psu, 16 gb ddr4, and a 1tb hdd. I'm currently looking into what I should upgrade first as it is starting to run really poorly and I am not seeing the performance that I would like(everything but the gpu is running slow). I play mostly fps based games such as siege, valorant, cs, and a little bit of fortnite. I have a 240hz asus tuf monitor, but I only get around 140 fps in game on everything but valorant. I have a budget of around 500 dollars currently and am looking for decent quality items with as little input delay as possible. Any thoughts as to what I should upgrade to, and what I should upgrade first? Thank you in advance.