On many PCs, there will be a few expansion slots on the motherboard. Typically they will all be PCI Express, but for a graphics card you need a PCI Express x16 slot. There are three versions of this slot, but they’re backwards compatible, so a modern PCI Express 3.0 graphics card will work in a motherboard with a PCI Express x16 2.0 slot.
Even if you have PCI Express x16 slot and plenty of room, you’ll need extra power for most graphics cards. Your power supply is likely to have PCI-E power connectors, but they may be bundled up and tied out of the way if no graphics card is currently fitted.
These connectors are usually black, marked as PCI-E and have six pins in a 3x2 arrangement.
If your PSU doesn’t have these, you can buy adaptors which connect to the standard four-pin power or SATA connectors. Be careful with graphics cards that require two PCI Express power connectors as each of these should be connected to a different 12v rail of the power supply. On most PSUs this means connecting each of the two adaptors to a different ‘daisy chain’ of power connectors, and not to the same chain.
How to know if a graphics card is compatible: Find the PCI Express slot