Do Witches worship the Devil?

The simple answer is no, but an explanation of this requires an understanding of the difference between Wicca and Christianity. The Devil is a Christian concept, essentially invented to explain the paradox of the existence of evil in a world supposedly created by a single all powerful benevolent God. The early church deliberately attempted to demonise other faiths, by the simple statement that all other gods/goddesses were either the devil or minions in his service. Often people were effectively forced to change their religion and pay homage and money to the church. The anti-pagan emphasis was made even more obvious by the adoption of the image of Pan as standard depiction of the devil (whose image is not mentioned in the bible).

As Wiccans do not believe in a single all powerful God they do not have any need, nor belief in the devil.

Witches are sensitive, intuitive people who work with the cycles of the moon. They are in tune with natures cycles in the form of the eight seasonal festivals of the year. They worship and reverence the Goddess and the Horned God, and have a deep regard and understanding for the mother earth and nature. Witchcraft includes a magical element, so Witches' cast spells using a wide variety of techniques and materials, but always obeying the wiccan rede.

Are there any Rules in magic?

The wiccan rede is "An' it harm none do what ye will". This positive morality advises against using the power of magic to do anything other than help anyone or anything. The price of braking this rule can be high.

The law of three fold return means that anything you do for good or ill is ultimately returned to with three times the power.

There is also the belief among many Wiccans that if a spell meets with a magical barrier it cannot reach then it rebounds on its caster immediately. This isn't a problem if the spell is intended to help, but could be very serious indeed if it was intended to harm.

Why do Witches use knives and what are they for?

A Witches knife is called an "Athame" (pronounced 'a-tha-me'), it is adouble edged knife, usually made by hand or altered by the witch. It is used for ritual purposes and to project energy. The Athame is a very personal item, you should never touch a witches athame, without first obtaining their permission.

Do I have to be in a coven to be a Witch?

No you don't if you prefer to work on your own. You will have just as much right to call yourself a Witch. Many chose this path, either from choice , or because of their isolated location. They may call themselves a Solitary or Hedge Witch.

What is the real meaning of a Pentagram/Pentacale?

The five points of the star signify the four elements (Earth, Air, Fire, Water) and the Spirit.