when you have the plant, dry it, after that extract with organic solvents (methylene cholored - Methanol ) (1:1) after that filtrated after 24 hour and thi concentrated and after that used chromatography methods for separation secondary metabolites.
Medicinal plant must be dried in shade to get good results, if not some compounds break down with hit of sun. For example some medicinal plants do have aromatic compounds and they will evaporate on certain temperature of sunlight after harvesting.
each and every medicinal plant have diffrent active compounds having medicinal value so first select prticulare plant and your target field .means pharmaceuticalor,agriculture .then which compound you want to produce and utilised for research.pls fixed your target.