A few specs, or links would help.
You probably have display ports, and or a HDMI.
You need an adapter HDMI to VGA, or Display port t VGA, or DVI to VGA
If you are just connecting a second monitor, a simple adapter should work.
If you are connecting a third monitor, both adapters have to be active, if using 2 display ports.
Some boards on-board video, is disabled, when you plug in a dedicated card.
Not all, but we don't know what your board is.
DVI to VGA, is the cheapest, and simplest.http://www.newegg.com/Product/Product.aspx?Item=N82E16812816053