Ash Wednesday is a Christian holiday (holy day) that is not a biblical requirement (rather like Christmas and Easter). Nevertheless, it has been honored by Christians for well over ten centuries at the beginning of Lent, a six-week season of preparation for Easter. In the earliest centuries, Christians who had fallen into persistent sin had ashes sprinkled on their bodies as a sign of repentance, even as Job repented “in dust and ashes” (Job 42:6). Around the tenth century, all believers began to signify their need for repentance by having ashes placed on their foreheads in the shape of a cross. Notice: even this sign of sinfulness hinted at the good news yet to come through its shape. Ash Wednesday is not some dour, depressing holy day because it symbolically anticipates Good Friday and Easter.
Mark has a good set of resources on Lent, as does Internet Monk. PCA Conversations has a good post. Reading Trevin Wax a few years ago helped encourage me to mark Lent.
Almighty God, you have created us out of the dust of the earth: Grant that these ashes may be to us a sign of our mortality and penitence, that we may remember that it is only by your gracious gift that we are given everlasting life; through Jesus Christ our Savior. Amen.