Here is an awesome place to start learning about Acala Vidyârâja. The text is taken from the Shingon Buddhist International Institute, and it is probably one of the best online sources to learn about Acala.
Learning the specifics about a deity's the appearance, symbolism and basic personality attributes is super-important in order to grasp the deity's basic notion behind the form. Every symbolism has spiritual meaning that is important to know about.
It is also good to start and form a clear mental picture of the deity in order to use in meditation, prayer etc.
ACALA VIDYARAJA or FUDO MYOO - THE IMMOVABLE RADIANT KING
Description Of Form Acala Vidyârâja is one of the Vidyârâjas (Myôôs) class of deities, and a very wrathful deity. He is portrayed holding a sword in his right hand and a coiled rope in his left hand. With this sword of wisdom, Acala cuts through deluded and ignorant minds and with the rope he binds those who are ruled by their violent passions and emotions. He leads them onto the correct path of self control. Acala is also portrayed surrounded by flames, flames which consume the evil and the defilements of this world. He sits on a flat rock which symbolizes the unshakeable peace and bliss which he bestows to the minds and the bodies of his devotees.
Purpose and Vows Acala transmits the teachings and the injunctions of Mahâvairocana to all living beings and whether they agree to accept or to reject these injunctions is up to them, Acala's blue/black body and fierce face symbolize the force of his will to draw all beings to follow the teachings of the Buddha. Nevertheless, Acala's nature is essentially one of compassion and he has vowed to be of service to all beings for eternity.
Acala also represents his aspect of service by having his hair knotted in the style of a servant: his hair is tied into seven knots and falls down from his head on the left side. Acala has two teeth protruding from out of his mouth, an upper tooth and a lower tooth. The upper tooth is pointed downward and this represents his bestowing unlimited compassion who are suffering in body and spirit. His lower tooth is pointed upward and this represents the strength of his desire to progress upward in his service for the Truth. In his upward search for Bodhi and in his downward concern for suffering beings, he represents the beginning of the religious quest, the awakening of the Bodhicitta and the beginning of his compassionate concern for others. It is for this reason that the figure of Acala is placed first among the thirteen deities.
His vow is to do battle with evil with a powerful mind of compassion and to work for the protection of true happiness. To pray for recovery from illness and for safety while traveling is to rely upon his vow and power to save. Acala is also the guide for the deceased, to help save them and assist them in becoming buddhas for the first seven days after death.
Nômaku sanmanda bazaradan senda makaroshada sowataya un tarata kanman. (Jpn.)
Namah samanta-vajrânâm canda mahârosana sphotaya hûm trat hâm mâm (Skt.)
Homage to the all-pervading Vajras! O Violent One of great wrath! Destroy! hûm trat hâm mâm
So, there you have it. You have the basics in order to start your practice with Acala or Fudo Myoo. You can make offerings (flowers, food, light etc) or meditate upon Acala's form and inner qualities, or recite the mantra. This practice can help you in ways that you can't even begin to understand.
Remember to enjoy the practice and dedicate your merit!