I have just got this new graphics card but it need a 8 pin pcie cable my psu only have a 6 pin there's a adapter in the box 6+6 to 8 pin do I need to put two 6 pin in the adapter or does work like a 6 to 8 pin adapter
If you must use an adapter (not a great idea), then yes, you should be using 2x6pins.
A single 8 pin is rated for 150W, a 6 pin only rated for 75W.
So if your card needs an 8, and you only provide a 6.... It's either not work (in this case, a good thing!) or it'll attempt to pull 150W from a cable rated for 75W. Not a good idea.
If your PSU does not have the appropriate cables for a GPU you intend to use, that's a good sign you need a new PSU.
Ouch. You're definitely well into the garbage-tier of power supplies here. The only 1070 this power supply should ever be connected to is a pile of 1070 other items waiting for processing at a recycling center.
The Corsair CX 550 is the least expensive PSU that I'd be willing to let touch a GTX 1070. Ideally, that's the level of power supply at which I'd prefer to go higher quality, like SeaSonic Focus or EVGA G2/G3/GS or Corsair RMx, but the CX 550 is at least a power supply worthy of the name.