What happened on the Seventeenth day of the Month of Tamuz?
a) The First set of Tables containing the Ten Commandments were broken by Moshe Rabbeinu on this date (Exodus 32:7 And the LORD said unto Moses, Go, get thee down; for thy people, which thou broughtest out of the land of Egypt, have corrupted themselves: 8 They have turned aside quickly out of the way which I commanded them: they have made them a molten calf, and have worshipped it, and have sacrificed thereunto, and said, These be thy gods, O Israel, which have brought thee up out of the land of Egypt... 15 And Moses turned, and went down from the mount, and the two tables of the testimony were in his hand: the tables were written on both their sides; on the one side and on the other were they written. 16 And the tables were the work of God, and the writing was the writing of God, graven upon the tables. 17 And when Joshua heard the noise of the people as they shouted, he said unto Moses, There is a noise of war in the camp. 18 And he said, It is not the voice of them that shout for mastery, neither is it the voice of them that cry for being overcome: but the noise of them that sing do I hear. 19 And it came to pass, as soon as he came nigh unto the camp, that he saw the calf, and the dancing: and Moses' anger waxed hot, and he cast the tables out of his hands, and brake them beneath the mount.)
b) The Two Daily Offerings were suspended on this date during the time of the First Bais Hamikdash
c) The Romans penetrated the walls of Yerushalyim on this date, three weeks before they destroyed the Second Temple
d) The rasha Apostumos burned the Torah
e) An idol was installed in the Bais Hamikdash

Why do we fast in the first place?
    We fast so that we might better come to understand that it is our actions, and the actions of our forefathers, that have led to these national tragedies.  If we can control ourselves enough not to eat or drink for an 18 hour period, we certainly can -- with an equal effort -- resist the sins that plague our everyday lives.  It's not just fasting, but it's returning to HaShem and His Torah, that will eventually lead to our return to the Land of Israel, and the universal recognition of the "Oneness" of HaShem:
--Jonah 3:10 And God saw their works, that they turned from their evil way; and God repented of the evil, that he had said that he would do unto them; and he did it not.
--Isaiah 1:18 Come now, and let us reason together, saith the LORD: though your sins be as scarlet, they shall be as white as snow; though they be red like crimson, they shall be as wool.
--Isaiah 55:6 Seek ye the LORD while he may be found, call ye upon him while he is near: 7 Let the wicked forsake his way, and the unrighteous man his thoughts: and let him return unto the LORD, and he will have mercy upon him; and to our God, for he will abundantly pardon.
--Ezekiel 18:21 But if the wicked will turn from all his sins that he hath committed, and keep all my statutes, and do that which is lawful and right, he shall surely live, he shall not die. 22 All his transgressions that he hath committed, they shall not be mentioned unto him: in his righteousness that he hath done he shall live. 23 Have I any pleasure at all that the wicked should die? saith the Lord GOD: and not that he should return from his ways, and live?

The Seventeenth of Tamuz begins a period in the Jewish calendar known as ohrmnv ihc ("between tragedies") or The Three Weeks (between the breeching of the Walls of Jerusalem and the destruction of the Temple).

During the Three Weeks, we observe differing levels of mourning, the most intensive mourning being reserved for Tisha B'Av.  The prohibitions for the first twelve days of the Three Weeks apply to the Seventeenth of Tamuz as well.  These prohibitions are:
1)  Regarding Engagements and Weddings 
--weddings should not take place during the entire Three Weeks even if a meal of celebration is not to be served; this prohibition stands even if one is without children and is getting married in order to attempt to fulfill this mitzvah (Genesis 1:28 And God blessed them, and God said unto them, Be fruitful, and multiply, and replenish the earth, and subdue it: and have dominion over the fish of the sea, and over the fowl of the air, and over every living thing that moveth upon the earth; Genesis 9:1 And God blessed Noah and his sons, and said unto them, Be fruitful, and multiply, and replenish the earth.).
--Engagements are permitted until Rosh Chodesh Av, even if a celebratory meal is served; from Rosh Chodesh Av until after Tisha B'Av, one is permitted to get engaged, but a seudah (celebratory meal) may not be served -- however, with the exception of on Tisha B'Av, refreshments, cakes and cookies may be served.
2)  Regarding Dancing and Musical Instruments:
--both of these activities are prohibited during the entire Three Weeks, with the exception of a musician who earns  living playing for non-Jews, he may perform in the home of a non-Jew until Rosh Chodesh Av.
--Practicing a musical instrument is prohibited on and after Rosh Chodesh Av.
--With regards to singing -- most Poskim allow that even during the Nine Days one is allowed to sing as long as not accompanied my music.
3)  Regarding the B'rachah "Shehecheyanu" ....