If you're a serious Japanese learner then you're going to need a quality dictionary early on.
There are more than 2000 basic Kanji in Japanese so you'll want to start out with a good Kanji dictionary (online or book). You'll find some of my recommendations below.
My first choice. A very handy, free online dictionary that includes Kanji and audio.
This dictionary is excellent and very popular. Also includes audio.
This a good one with audio but the interface is quite old and outdated.
A very detailed word lookup tool from Monash University. Very outdated interface however (and no audio).
Extremely basic tool for looking up words with limited output. Handy though.
One of the highest quality Japanese dictionaries I've seen, but lacks audio.
A handy tool for finding word definitions and translations (but very slow).