As much as possible try to use a true Spanish dictionary (such as http://www.elmundo.es/diccionarios/ ).
If your goal is to be a fluent Spanish-speaker then you have use a Spanish-Spanish dictionary! That's what native speakers do.
A good online Spanish/English-English/Spanish dictionary is: http://wordreference.com/ That's a great reference for spelling, gender of nouns, different possible uses and meanings; in other words, it is a good reference for language learners. But fluent speakers would use the Spanish-Spanish dictionary a lot more.
Try to find a way to use both!