Japanese is a phonetic language and its Romanization (Japanese written with English letters (ABC's)) corresponds very much to the way it should be pronounced.
section of this website contains some information on aikido terms. Also, if your browser and computer handles sound files, you can check out the Aikido Audio Language Files
here on AikiWeb which will provide you with audio files of a lot of common aikido terms being pronounced.
Most likely what is being said at the beginning of class is "onegai shimasu" which can be very loosely translated as "please train with me." The aforementioned Language
section has a more in-depth article on the term
Hope that helps,