Summer plants: plant out after last frost in April or May.  
 
Summer squashes 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 18" apart.
 
Variety information:
Yellow straightneck squash: Delicious yellow squashes on fast-growing plants.
Zucchini: Classic straight, dark green zucchini squashes on robust plants.
Pattypan: Nutty-flavored, pale green squashes shaped like flying saucers.  Taste is more similar to yellow squash, although the texture tends to be firmer, like zucchini.  Great for grilling and frying.