Hi
You need a 6 pin and a 8 pin, the 8 pin may be a 6+2 like you mentioned.
So you need to connect the 6+2 into the 8 pin socket and you then need an additional 6 pin cable.
If your Power supply doesn't have an extra 6 pin cable you will need a molex to 6 pin adapter which may have come with the graphics card as part of the accessories.
Just a warning: if your power supply doesn't have the correct cables it may not be powerful enough for your graphics card, do you know which make and model the power supply is?