Dahlia tubers should be planted after all frost has passed. The soil needs to be worked and compost or other organic matter added in so it's loose and fertile. Some gardeners add bone meal before planting. A 1/2 cup or so for each tuber works well. Work it into the soil before planting. Dahlias should be planted in full sun, and kept watered but never soggy. You'll also need to keep the bed weed free because they are heavy feeders which means they don't share well. Raised beds work very well for Dahlias.
Tubers such as dahlias are planted by laying the tuber on it's side with "the eye" facing up. This is where the stem will shoot up from. You should be able to see this be examining the tuber root. A depth of 6 inches is usually recommended. Don't pack the soil on top too firmly, but fill it back in gently and water.
Unless you are planting dwarf Dahlias, they most likely will need support, especially the larger varieties. Plan ahead by putting in supports as you plant. Dahlias also work well planted in containers. They are beautiful old fashioned flowers that come in many sizes and colors. I've grown dwarf dahlias in porch boxes with great success. Purchase them online or at local garden centers. I often buy the dwarf dahlias as bedding plants in early summer.
On OFL we have tips on growing zinnias which are similar to Dahlias but they are very easy to grow from seed.