Summer plants: plant out after last frost in April or May.  
 
Cucumber plants are sold in 2.5-3" peat pots or plastic pots.  Peat pots can be planted directly in the ground, but I recommend tearing strips in the sides and bottom to allow roots to quickly grow into soil.
 
Space plants about 12-18" apart.  Train onto a trellis or allow to grow across the ground.  Vines of most varieties will spread up to 8' around the base of the plant if left on the ground.
 
Variety information:
Marketmore: Long-time favorite for long slicing cucumbers.  Great for salads, fresh eating, and making sliced pickles.
Supremo: Newer hybrid variety that we like for its resistance to disease.  Crunchy "pickling" sized fruits that are great in salads, eaten fresh, or turned into full-size pickles.  These cukes should be harvest when small.
Spacemaster: Medium-sized cucumbers grown on bush-type plants that do not send out long vines.  Great for tight spaces where you don't want a trellis, or for growing in a pot.