Before having food, the following mantras need to be chanted..

1) OM BHUR BHUVAH SUVAH
TAT SAVITURVARENYAM
BHARGO DEVASYA DHIMAHI
DHIYO YO NAH PRACHODAYAAT

This is called Gaayatri mantra - gaayantam trayate iti gayatri - ie, that which protects it's chanter is gayatri.

2) take some water in hand, and spill it over the place, enrircling the food in the anticlockwise direction.

while doing so, chant the Gaayatri.

3) then take small bits of food each and put it in your mouth.
Each time, chant the below mantras.

OM Pranaaya Swaaha
OM Apanaaya Swaaha
OM Vyanaaya Swaaha
OM Udanaaya Swaaha
OM Samanaaya Swaaha

Finally, OM Brahmane Swaaha
and then Brahmana AAthma Amritathvaaya.

4) Start taking full morsels of food. While eating food, it is desirable that you keep in mind God and His Vibhutis. (what I do is, ponder over the meaning of the holy names in Vishnu Sahasranaam)

5) At the end, sip a bit of water, enough to wet your throat, and chant the below mantra.

Amrithaapidhaanamasi. (let this food confer immortality to me)

If you require further clarifications, let me know.

Sahasraarchi