That's a tough question! You haven't told me where you live. This is important for a number of reasons. Most importantly, plants that are hardy in one area of the country are not hardy in other areas of the country. A perfect example are salvias. I love salvias, but I have to grow almost all of them as annuals in my zone 6 NJ garden because they are not hardy here. In southern parts of the country all types of salvias are hardy and grown as perennials.
How much sun does your garden get each day? "Full sun" is defined as a MINIMUM of 6 to 8 hours of sun each day. If your garden only gets sun in the morning or in the afternoon, that is considered partial shade. Plants requiring full sun will be stunted in partial shade and most of them will refuse to bloom.
What is your climate like? Is it arid like the southwest? In that case, you will need to plant drought tolerant plants. Is it wet like the Pacific Northwest? Then you will need plants that can tolerate lots of moisture.
What kind of soil do you have? Most plants will not grow in clay or sandy soils but I can't tell you how to amend your soil if I don't know what kind you have.
How large is your garden? Smaller gardens are best planted with smaller plants while large gardens can handle large plants.
Your best bet is to visit your local nursery. They will know which plants are perennial in your area and what kind of soil you have and how to correctly amend it.
After you install your plants in your garden, make sure that you water, water, water. Summer is the worst time to be planting perennials. They should be planted in the spring or the fall when it is cooler and less stressful for the plants.