plz help me!!! im only 13

It's very hard to understand what you want. If possible, please try separating your statements into sentences by using periods as appropriate.

I think I understand a little bit, but I'm not sure. You think you need to upgrade your graphics card because it isn't performing very well while you are playing Halo. You bought a new graphics card to fix the problem, but you don't know how to open up the case and install the new card.

So, are you wanting to know how to open up your computer case so that you can install the new card?

to scan your PC use CPU-Z
then click the mainbaord tab, under Graphic Interface, you'd see whether you Motherboard supports AGP or PCI-EXPRESS

if you still want more infomation, come back and tell us the new card you bougth and your PC brand & model, we can help you much more with that