Dream dictionaries can be extremely useful in aiding you through the interpretation process. That said it's important to remember that dream symbols and themes are relative to the dreamer. It would be foolish to assume that all symbols have the same meaning for everyone. For example, imagine you had a phobia of cats. If you have a dream containing cats it will likely have a very different meaning to a cat-lover who had a similar experience.
To get the best results out of a dream dictionary, it is best to examine your relationship with the symbol before searching for the term on the Internet. Take a piece of paper and write down the symbol's name in the middle of the page. Around this start noting anything that comes into your mind. Do not use your discretion! It is important to allow things to flow as you will likely produce some very interesting ideas as you continue on with the process.
Once you are finished consider showing your work to a close friend and ask them for any ideas on what it could possibly mean. Remember, you know yourself better than anyone else so it's best to analyse any external input before using it to come to a conclusion.
Listed below are a number of reputable online dream dictionaries that I cannot recommend enough.
Dream Moods
Dream Moods offers an extensive dream dictionary which is sorted both alphabetically and by theme.
Sleeps.com
A vast A-Z guide that accepts and reviews user submissions.