A lot of beginners like to use
Jisho.org: Japanese Dictionary , which is a pretty nicely laid out site.
The database it uses comes from
WWWJDIC: Word Search , but most people seem to prefer jisho's interface.
Personally I prefer to use a site like
国語・英和・和英・類語・中国語辞書検索 - goo辞書 , partly because I like the example sentences in the Japanese->English dictionary and partly because I like having the regular Japanese dictionary right there. Of course a beginner can't really read definitions of words in Japanese so that's not a consideration until later.
A lot of beginners apparently find it hard to navigate around a Japanese website like this though. I'm not sure why... you put your word in, you click the search icon, then even if you can't read the word 和英 you still can click on the entry that includes English. If you're searching in English the only result of course is from the English->Japanese dictionary.
I would recommend reverse-lookups too... after you look up the English->Japanese, make sure to look up the Japanese->English to help clarify the meaning and make sure that you have the right sense of the word.
Oh, and if you can afford it you could probably buy an electronic or paper Portuguese->Japanese dictionary. (Seeing as you're in Portugal, I'm assuming that's your native language.) I'm not aware of any free web services though for Japanese dictionaries other than Japanese->Japanese, Japanese<->English, and Japanese<->Chinese.
