Ganesh is the Lord of obstacles, the one to hail at the beginning of endeavors, who gives knowledge and wisdom, and who protects (averter of evil!).
For the celebration itself, we turn to the holy wiki:
Ganesh Chaturthi starts with the installation of these Ganesh statues in colorfully decorated homes and specially erected temporary structures mandapas (pandals) in every locality. The pandals are erected by the people or a specific society or locality or group by collecting monetary contributions. The pandals are decorated specially for the festival, either by using decorative items like flower garlands, lights, etc. or are theme based decorations, which depict religious themes or current events.
The priest, usually clad in red silk dhoti and shawl, then symbolically invokes life into the statue by chanting mantras. This ritual is the Pranapratishhtha. After this the ritual called as Shhodashopachara (16 ways of paying tribute) follows. Coconut, jaggery, 21 modakas, 21 durva (trefoil) blades of grass and red flowers are offered. The statue is anointed with red unguent, typically made of Kumkum & Sandalwood paste . Throughout the ceremony, Vedic hymns from the Rig Veda, the Ganapati Atharva Shirsha Upanishad, and the Ganesha stotra from the Narada Purana are chanted.Speaking of chants, here are a few favorites:
"Vakratunda Mahaakaaya Suryakotee Sama Prabha Nirvighnam kuru mey Deva, Sarva kaaryeshu Sarvadaa"
O Lord Ganesha of Large body, curved trunk, with the brilliance of a million suns, please make all my work free of obstacles, always.
"Aum gam ganapataye namah"
Om Salutations and Prostrations to Lord Ganesha
Here's a nifty web page with more chants, their meaning and how to use them, along with advice on ritual cleanliness, repetitions, etc.
I really like this video. It reminds me of the video I posted for the feast of Our Lady of Mt. Carmel! (the sound on the video isn't good, I suggest watching it on mute)
Ganesh Chaturthi by WildFilmsIndia
Ganapati Bappa Moryaa!