There are guidelines on choosing words from Teresa Carey, charter player and original Rules Mother. Bear in mind that this was written rather a long time ago and many fun and successful rounds have been played that did not follow the advice offered here.
Below are links to online references to help you find words and dream up definitions. If you use a dictionary to help compose a definition, you may inadvertently disqualify yourself from voting. If you do this, you can still submit a definition, but you reduce your chances of scoring well in that round. If you don't understand why it may be inadvisable to use a dictionary, read the rules of the game.
Bilingual Dictionaries - Browsable lists, other-to-English where possible. Sites that offer only a find-the-word interface have been passed over; even if more comprehensive, scholarly, etc. A deliberately off-beat selection of mostly Indo-European languages.
English Dictionaries - A broad range of dictionaries and dictionary sites, old and new, including dictionaries of synonyms and etymology. These resources are free, but some of them provide limited access to encourage you to sign up for a premium service.
Specialized Vocabularies - English dictionaries and glossaries devoted to individual occupations, sports, crafts, sciences and technologies.