Sockets for graphics card in motherboards have change a lot over the years.
They used to be AGP (Accelerated Graphics Port) slots but these began to be phased out from about 2004.
They were replaced by PCI Express slots.
So you need to find out what type of slot you have in your motherboard, AGP or PCIe.
Nearly all graphics cards nowadays are PCIe and it is hard to find an AGP graphics card (unless you buy second hand).
You don't say what operating system you are running, but you whatever graphics card you buy you need to make sure it comes with drivers for that version of Windows.