Hey! I grew up in Brooklyn...back when they had trees.
it's going to depend on whether it's a sunny location or not. Some plants just grow better in the sun...most flowers like the sun. Eventually, the tree will supply shade to the stuff growing at its base, but you probably don't have to worry about that for a while.
Pachysandra....not really flowery but a good sturdy choice. easy to grow and hardy.
Creeping Phlox... aka Mountain Pink....low growing ground cover flowers are pink to purple in the spring.
Cosmos...flowers in a variety of colors right up to first frost. Choose carefully, these come in many varieties with some types growing to 4 ft. tall. A good low-maintenance choice.
Remember that perennials grow back every year and annuals only grow for one year. You can experiment with annuals and try something new each spring. Check out a few websites for pictures and you can find a local garden center for their advice. Bring a picture.