Heating a small 6x8 greenhouse

Hello everyone,  I am new to this board, and, also to building a small green house.  It was from harbor freight, the 299.00 6x8.  I am going to place it on a concrete pad that is in my yard and also increase headroom by building a base with 4x4's or 2x6's.  I live in NJ and would love to extend my growing season by heating my greenhouse.  I am in no means rich, so I need a way to heat it by either solar panals of some type of using a water system to heat.  HELP !!!!!!!!!!