A dictionary is the essential tool when learning a language. There are many dictionaries to choose from. I will give you some tips to find the dictionary for your needs.

Determine Your Proficiency Level
It is important to know your proficiency level and your goal when choosing a dictionary. Compact dictionaries will be handy for the classroom or trips, but you might need a larger one for your continuing studies. Though a larger dictionary can be expensive, it will be a great investment.

Monolingual or Bilingual
It is ideal to have both a monolingual and a bilingual dictionary. Beginners probably get by with a bilingual dictionary at first. However, a monolingual dictionary will be a great help to improve your ability to think in Japanese without translating words.

Furigana or Romamized 
With furigana, the beginner who is familiar with hiragana and katakana is able to read all Japanese writing. If you are at this level, a furigana dictionary will be easier to use, and helpful for further studies. A romanized dictionary is useful for students using textbooks that are written in romaji. Even complete beginners with no knowledge of Japanese writing will be able to use it easily.

Kanji Dictionary
Kanji is one of the hardest parts of the Japanese language. A kanji dictionary is a great tool for anybody who is interested in kanji. It usually includes meanings, pronunciations, stroke order, common compounds etc. 

Online Dictionaries
There are various free online dictionaries available. Personally I like to have my own dictionary, but online dictionaries are a free and convenient alternative.