10 gears to singlespeed
I've got a 2nd hand Hercules too and I've done this exact thing. Get a quality chain tool and look up some videos on Youtube about how to remove and re-link a bike chain. I pretty much winged it as I had no clue what I was doing. It took a bit of work and a lot of frustration to get the chain length right. I probably did it wrong, but it works. 

EDIT: Argenti Apparatus mentioned the dropouts.. One thing i did to get the chain tight was loosen the rear wheel from the frame, and pull it back as far as i could pull it.