Study the history of the Orcs. This has in fact been the subject of considerable scholarly debate, as it appears Tolkien was not himself fully resolved on this matter. It is generally said that the Orcs were made by the god-like but corrupt Melkor (sometimes later referred to as Morgoth) from Elves that he captured and perverted. Though the means by which this was accomplished are not clear, it seems the Orcs became a race unto themselves. It is possible, therefore, that their language is also rooted in one of the Elvish dialects. This suggests that a study of the Elvish dialects is a good place to start.
Search through the texts of Tolkien's Lord of the Rings trilogy for passages that contain Orc speech. These can be found in the texts using indexes and glossaries. This will give you a fairly good overview of the vocabulary for while it is stated in various places in the Tolkien works that Orcs have their own language and that the language is diversified into dialects, it is also clear that Orcs do not have a lot to say. Their speech is suitable primarily for uttering abuses and other hateful comments, or talking about violent things or base functions.
Study the Orcish language passages in the text. Use glossaries for help with pronunciation, and practice reciting the passages aloud.
Commit the Orcish passages you can find to memory. This will enable you to call up useful phrases in Orc speech at the drop of a hat.
Practice your memorized Orc phrases regularly so you they will easily come to mind. Remember that "use it or lose it" is a rule in learning a language. Using the Orc words and phrases is in itself a good way to practice. Chances are no one around you will know what you are saying, so this is a good language with which to say insulting things without causing problems with your coworkers, family members and others.