A good monolingual dictionary is a must for learning German. (Monolingual means that this is a German dictionary, and the definitions are in German.) Here are a few to choose from:

1. Duden Volume 1: A good standard dictionary. 115,000 entries, 800 pages. Available in the U.S. They publish 12 volumes, but volume 1 is the one you want.

2. Brockhaus Wahrig: Six volumes, 220,000 entries, 85,000 technical terms.

3. Grimm Deutsches Wörterbuch: 33 volumes, 34,000 pages! This is the German equivalent of the Oxford English Dictionary.

You should probably get the Duden. It’s available in the U.S. (old used editions are very cheap).