Salt, with its preservative properties, had always been treasured as precious 1 in the ancient world, and seen as a symbol of incorruption and wisdom. It's use was commanded by God --
Whatsoever sacrifice thou offerest, thou shalt season it with salt, neither shalt thou take away the salt of the covenant of thy God from thy sacrifice. In all thy oblations thou shalt offer salt. But if thou offer a gift of the firstfruits of thy corn to the Lord, of the ears
-- and it was seen by God, as recorded by Moses, to act as a symbol for that which can't corrupt:
All the firstfruits of the sanctuary which the children of Israel offer to the Lord, I have given to thee and to thy sons and daughters, by a perpetual ordinance. It is a covenant of salt for ever before the Lord, to thee and to thy sons.
Its first recorded sacramental use was by Eliseus (Elisha) to restore waters of a well:
4 Kings 2:19-22
And the men of the city, said to Eliseus . Behold the situation of this city is very good, as thou, my lord, seest: but the waters are very bad, and the ground barren. And he said: Bring me a new vessel, and put salt into it. And when they had brought it, He went out to the spring of the waters, and cast the salt into it, and said: Thus saith the Lord: I have healed these waters, and there shall be no more in them death or barrenness. And the waters were healed unto this day, according to the word of Eliseus, which he spoke."
And, of course, there is Our Lord's calling His people "salt of the earth" and warning of salt that loses its savor (Matthew 5:13, Mark 9, Luke 14), and there is St. Paul's warning in Colossians 4:6 to "Let your speech be always in grace seasoned with salt: that you may know how you ought to answer every man."
Salt is now used sacramentally in two main ways in the Church.
First, in Baptisms: like the baptismal waters, salt is blessed and exorcised. Then it is put on the tongue of the catechumen during the Baptismal Rite.
Second, for use in the preparation of Holy Water and for the use of the faithful: regular salt is exorcised and blessed and is used in the preparation of Holy Water. It is also given to the faithful for their everyday use -- e.g., for use in cooking; for sprinkling around rooms, doorways and yards, to protect against evil, etc.
Because of its exorcism and blessing, it is a powerful sacramental in keeping away demons. To obtain blessed salt, just take ordinary salt to your priest and ask him to bless it.
Note: salt -- usually not blessed -- is also used to purify the priest's fingers after Unction.
1 Salt was considered so valuable that Roman soldiers were paid, at least in part, by salt, or "sal" in Latin. This is the root of our word "salary."