You don't need to be able to read the japanese characters to be able to understand spoken Japanese. But if you understand written japanese without knowing the Hiragana and Katakana, i'm really surprised! xD
Even if it's just for speaking, learning at least one of these will help you alot with recognizing the sounds and with pronounciation.
If you want just need to be able to read to get by, and not want to learn more than a little japanese, you might want to start with Katakana, since all loan words are written in katakana. So with a little imagination, at least you can understand the "engrish" in japanese ^^
If you are aiming higher however, learn the Hiragana first, it's the most important one :)