Do a wet/dry compression test. If the dry test is low on one cylinder, and the wet is ok, you may have bad piston rings (expensive). If the dry test is low and the wet is low, you may have bad valve stem seals (5/10 difficulty if you have some tools and a shop manual), if you are low between two cylinders, or see discolored residue on one of your spark plugs, you may have a bad head gasket (7/10 difficulty on a home-job).
If you are unsure about which, get a mechanic, or mechanically inclined friend to advise you. In hondas, it is often cheaper to get a new motor second hand unless you can do the work yourself.