As for the parsley you should put it in a place with full sunlight, and lots of it, at least 5-8 hours per day. If you have a sunny window sill with southern exposure, that should be perfectly sufficient full sun light in the summer months. In the winter, you will need supplemental light no matter what. it is necessary a container that is both wide and deep (a long window box planter is an ideal choice).
According to my experience, what makes the difference is the soil, you should use a mix that is light and contains a mix including perlite or vermiculite, coir or peat moss, and compost.