The most environmentally friendly way is to dig them out.  Dandelions have a long tap root.  You may be familiar with the 'dandelion digger'.  It is a straight metal rod with a notch at the end.  It is put straight down into the earth and used to pop the plant out.  Garden trowels could also be used.  Shovels are a big over-kill.  Otherwise a selective herbicide.   Commonly available homeowner products contain one or more of the following active ingredients: 2,4-D, 2,4-DP, MCPP, MCPA, dicamba, triclopyr, carfentrazone, sulfentrazone, quinclorac. Make sure that the product is labeled for lawns.

When used following label directions, herbicides recommended are safe for the environment.  You need to SPOT SPRAY.  You do not spray willy nilly.  Spraying is done on a mostly calm day and directly onto the center of the plant.  We do not recommend products by brand names.  Go to a full service garden center and read the label.  Find one that is labeled for use on dandelions.