After creating this form , Ganapati asked him to worship Lord Shiva and taught him the mantra "Satarudriya" , he recited "Satarudriya hundred times and Lord Shiva Got pleased with him and asked him to choose any boon he liked .
This man asked a boon that half of his opponents power in the battle will be united with himself and his arrows will make the opponents missiles and weapons futile. Lord Shiva granted the boon after thinking and said "Bhanda-Bhanda".
And this man created by Citrakarma is came to be known as Bhandasura. Since Bhandasura was borned from the fire of anger of Rudra he became Danava of dreadful nature.
Sukracharya crowned Bhandasura and He presented him an umbrella formerly made by Brahma. himself. People seated in its shade were never affected by even crores of missiles. Alo given him bow named Vijaya and a conch called 'the killer of enemies.
Bhandasura was very keen on worshiping Lord Shiva and conducted all vedic activites for sixty thousnd years. After that time has passed. And after seeing the increasing power of Bhandasura over Indra Lord Vishnu created Maya (Mohini) and asked her to reside inside Bhandasura. After that Bhandasura got possessed by Maya , he forgot the Vedas and lord Siva too and forsook Yajnas and other auspicious rites.
Then Sage Narada went to Indra and said to him that if Bhandasura was liberated from Maya of Lord Vishnu ,he will burn all the three worlds like fire and its necessary to kill that demon. This task can only be carried out with the Adi ParaShakti ParaShakti ( Supreme Goddess , Lalita)( Supreme Goddess , Lalita) . Indra then went to Himalaya and praised Lalita with Hymns along with great worship. Adi Parashakti manifested as goddess Lalita and married Kameshwara (Lord Shiva)(Shiva+Shakti always together).
Lord GaneshaLord Ganesha destroying "Jayayantra" and killing "Gajasura"
So according to Brahmanda Purana Goddess Lalita granted the this boon of right of worshipped before the worship of all other deities to Lord Ganesha. The whole interesting story is in between chapter 11 to 30 of Lalita-mahatmya of Brahmanda Purana.
This post also provides some insight about why we worship Ganesha before starting any auspicious work , Venture ,Puja etc. i.e. to remove obstacles coming in the way of that work and to get blessings of Ganesha about growth and prosperity in the same.