The first thing you will need to do is have holes in the bottom for drainage. You don't want water sitting in the bottom, keeping the soil wet as that will cause fungal problems.
For shade I love impatiens. Here's a site that shows lots of color combos.
Caladiums have lovely leaves and, with the right selection, you can add them to your container with the impatiens.
Coleus have lovely leaves and, if you pinch them to keep them full and keep off the flowers, you can bring them indoors and keep them going for the winter. You also take cuttings and root them in water to grow indoors fot the winter.
Torenia is a lovely flower. I perfer the darker colors.
To have something cascading over the edge try some ivy. Many different leaf shapes and some are variegated.
Purchase a good potting soil, preferably one that doesn't contain fertilizers. Organic fertilizers like fish emulsion and seaweed are wonderful for plants in containers. You could mix the two together or you might find it already mixed.