Next step

Hello. I’m looking for some advise. I bought a basic computer online a while ago. (I think I’ve posted here before) never owned a PC in my life and don’t really know what I’m doing. I want to upgrade my PC as much as I can really. Don’t really play a lot so not wanting to spend loads. What’s good enough to stay in for a while and what’s the 1st thing you would change? The tower itself isn’t massive so not sure what I can and can’t get in it. Any help is appreciated.

Gaming Desktop PC (i5-3570K, Radeon R9 280, DDR3 24GB, SSD 240GB) Specifications:

Case: Gaming Mid Tower

PSU: Corsair 450W

Motherboard: Gigabyte GA-Z77X-D3H

CPU: Intel Core 15-3570K Quad Core PCle

Graphics Card: AMD Radeon R9 280 3GB

RAM Memory: DDR3 24GB

SSD: Kingston 240GB