Ninahitaji is the literal meaning of the words "I need". Whereas ninaomba is the word for "i request"
Is this how you would ask for food in Swahili?
According to what I've seen in phrasebooks, (ni)naomba XYZ is better.
No, tafadhali is basically used in Kenya and on Zanzibar; in most of Tanzania, "Naomba" is better and politer.