First of all you need to look up your zone for sure. I don’t think you are a 5. Then you need to decide do you want them to come back every year or not. The ones that come back are perennial and the others last only one season and are called annuals.
You can do a combo of both. Plant some that come back every year, like tulips, iris etc. then around the bottom plant some that are ground cover they will cover the dirt close to the bottom of your other flowers. Early annuals that you can plant when still kinda cool are pansy, then you could add petunia. Both of this continue to bloom and are pretty easy to take care of.