Answer: Hillside gardens can be stunning, however they are difficult to plant for a number of reasons. When you work on your yard, pay special attention to the way it drains so as not to disturb the existing drainage patterns. This is important in terms of protecting your house foundation from excess water -- the property should be carefully graded to direct water away from the building(s) and disrupting that can have unintended consequences. You may want to consult with a professionally trained and degreed landscape architect for assistance in designing planting areas for this reason.
You would begin by considering plants that are winter hardy in your area. Your zip code places you in zone 6A or the coldest part of USDA winter hardiness zone 6. In a rural, exposed, or windy spot it could actually be as cold as zone 5. For this reason I would suggest you use plants considered hardy to zone 5 or colder.
As far as plant selection, you will need to analyze the growing conditons in each area and select plants suited to those specific conditions. Lighting, soil type, wind exposure, space available, and desired "look" will all influence your decisions.
Hillsides tend to be well drained due to the slope, so you may need to work specifically with plants that tolerate a well drained location, potentially a dry one in summer, depending on the soil type.
If there are native plants in nearby areas similar to your yard, these will give you clues as to what might do well in your yard in similar conditions -- ie exposure, slope, soil, type of lighting.
Your local county extension should be able to help you with soil testing to check the soil fertility and pH levels, as well as have suggestions as to plants that would do especially well for you. You may also find the search features at the following web sites helpful in beginning to identify plants that might grow well for you:
In the meantime, I hope this helps you begin thinking about what to do.
Q&A Library Searching Tips