That computer doesn't lend itself to upgrading...
So it has a motherboard with only 2 DIMM slots, so you'll always be limited to how many sticks of RAM you can throw in.
It has a 1155 socket, so you can't upgrade to the latest Intel processors
It comes with an extremely weak PSU (90 watts), so you can't put a big video card in it (even if you could fit it) - you're going to have problems powering anything
The case is small and poorly ventilated; it can't fit a big video card; there is no space to put a radiator; I don't even think there are empty drive bays (you can't install additional disk drives or optical drives); its cooling potential is limited...
Basically, I would advise you to start over. Don't mean to rain on your parade, but of the most basic upgrades:
1) You want to upgrade the CPU? You need to buy something that uses only 35 watts, because your PSU and mobo can't handle higher. Most of the bigger chips these days are 55ish watts. And you need to buy older chips (1155 socket)
2) You want to upgrade the GPU? Probably not enough power to run it, and probably not enough space in that case
3) You want to add an SSD...you can swap the HDD for an SSD, but that's not really efficient. Usually, people have smaller SSD boot drives and then big HDD storage (where you don't need the access speed, and the storage is cheaper) - you can't because you're limited both in power and bay space
4) You want to add RAM? You can do that - but only one stick.