I should have 1 Gbps, but I'm downloading at about 8 Mbps. Why, or what can I do?

My internet provider has three different plans, and we have the 1 Gbps plan. However, my PC only downloads at 8 Mbps, which is very slow. As far as I know, our house has a fiber optic connection. I'm using a LAN cable, of course. Can someone help me and tell me what I can…