there's a cord thats fat/black and looks like it gets connected to the mobo but I dont know what its used for, I plugged in everything else include fans the lights but donr know what that thick cord is. can someone explain/
Can you describe it a bit better i.e.,what do the ends look like, how may wires/pins.
Is the power cord plugged into the power supply?
The other thick cord is usually the main power connector for the moBo( hard to miss that one)
With the exception of the fans, all other cords get plugged into the appropriate sockets ion the mobo. Some case fans can be plugged into the mobo fan headers to enable the mobo to control the speed, or they can be directly daisy-chained to the 4-pin Molex power plugs connected to the PSU.