Doug there is still time to plant bulbs for early spring bloom. Daffodils, tulips, cocus etc. need to be planted in the fall.
Annuals can be planted in the spring after the danger of frost ( usually around mothers day) and will bloom until fall - then the die and need replanted.
Perennials can be purchased now and planted now ( soon) and will bloom for a period of time ( 3-4 weeks on average). Flowers will die, but plant will remain and live through winter and bloom next year.
You have not said if you have sunny location or shade, what the soil is like - All will impact the type of plants we can suggest for your site. I.e Hostas like shade , Blackeyed Susan likes sun....
Pittsburgh should have a few nice garden centers. Start there and you will learn from the staff move about your plants and what does best. I like HD and Lowes, but in some stores you may not be able to find a person that can help you with more detailled knowlege of the plants.

I attached a link to my blog. You may find some information that will be helpful - Mike

Here is a link that might be useful: Garden Design Ideas