What's the easiest way to add gears to my single speed? I recently bought a single speed - a Charge Plug Racer, and I realise I've made a mistake - too many hills on my commute and I need a smaller gear to help me up the hills. It's pretty hilly in my area.

I still like the simplicity of the bike, but cannot get up all these hills!

What's the easiest way to add a gear?
Depending on what you want, you might just be okay lowering the current gearing.  It comes with a 42t chainring and a 16t freewheel, and switching to a 17t or 18t freewheel will decrease your overall top speed somewhat but make it easier to climb up those hills.  If you want to have the option to swap for a harder gear when you're planning a mostly flat ride, you could either install a different sized freewheel in place of your fixed cog (on the flip side) or get something like a White Industries DOS 2-speed freewheel.  In either case, you'd change gearing by loosening the rear wheel and moving the chain position by hand.

If you want to switch to a true multispeed bike, you've got a couple options. Easiest might be to purchase a new wheel built around an internal gear hub.  Your rear dropout spacing is 120mm, and there are a few hubs that come in or can be reduced to near that width - notably the Shimano Nexus 8 Red Band, the SRAM P5, and several of the Sturmey Archer hubs can be made to work.  You could also look into getting the frame cold set to a wider dropout spacing, which would open up more IGH options (the Alfine 8 being a great one) as well as conventional multi-speed cassettes - though you'll need to get a derailleur hanger chain tug, like those made by DMR.