Sionned is right - arna i is basically a personal form of the preposition "ar". Welsh does this a lot, and you may have already come across it with the "amdana fi / amdanat ti" construction. What is happening here is the preposition "am" is being personalised. In the case or arna i, it is "ar" being personalised, or "on" in English. The construction (I believe) is a very old Celtic way of phrasing - so you're saying "mae eisiau bwyd arna i", or literally, "there is a need of food on me", or "I'm hungry", essentially. Again, corrections from more advanced Welsh speakers welcome! Don't be worried if this sounds complex - if you learn it all by ear, you will notice a simplicity in the way it works, and you'll remember it easily.