You probably need a 15 mm wrench to unscrew that spike from the crank, with a pipe to act as a prybar on the wrench. First let some Liquid Wrench penetrating oil soak into the threads overnight. Since pedals come in pairs you should also remove the left pedal - that one has to be unscrewed clockwise. Take the pedals to the bike shop and get a new pair with the same size threads. Put some grease on the threads before you install them so you dont have too much trouble taking them off later.
I think you would find it cheaper and easier to find a better quality used bike than to upgrade this one - ask about used bikes when you take the pedals to the bike shop - good luck!