Can you put it into download mode? If you can then you should be able to odin a custom recovery or odin the stock rom back to your phone and start from scratch. Have you downloaded the Samsung USB drivers? Only use the cable that came with your phone, others may not work or charge only. Also, try a different usb port.
Signature fails because you are on the stock recovery and the file you are trying to install hasn't been signed by Samsung.
Phones & Tablets: Galaxy S4, Galaxy Note 8", Evo 4g LTE, Evo 3D, Acer Iconia A500, Samsung Galaxy Tab 2 10.1, Ipad 2, Nabi 2,
Rom: Rooted Stock for now, Hotspot Mod, Xposed, Wanum
Kernel: Stock for now