You must learn hiragana as a first step to learning Japanese.
Sure, you can get by just by using romaji, but romaji has many problems. Here are two big ones.
Firstly, your pronunciation will be terrible and is unlikely to ever improve. Why? Because you are reading roman letters as you do in English and you will constantly be fighting yourself not to use English pronunciation.
On the other hand, if you learn to read the kana, you pronunciation will be good and will improve rapidly.
Secondly, not learning hiragana and relying on romaji means that you will not be able to easily find information in Japan. If you learn hiragana and katakana you’ll be able to get around in many places while also improving your language skills.
Both hiragana and katakana are very quick to learn. You can find out how to learn them both super fast at http://learnhiragana.net