There are some free online courses that you can find by doing a search on the term Learn Mandarin. Some of them have audio files so using the material on those sites should give you an idea as to how Mandarin sounds.
For example, there's:
- BBC's Real Chinese (which is a site related to their TV show with the same name)
- Chinese Tools (31 lessons)
IMHO, it's a good idea to start your learning process with some one-on-one time with a native speaker because the accents are particularly tricky and even if you can hear the correct pronunciation, you'll still need someone with a good ear to correct you when you're first starting out.
If you're allowed, bring a tape recorder to those one-on-one sessions because you'll need to keep listening over and over to get the hang of it.
If you do get lessons, you'll most likely be taught how to pronounce pinyin (the romanized version of the spoken language) properly. If so, you can build on that foundation and learn new words online -- many sites now include pinyin next to the traditional chinese character.
Note that speaking Chinese is vastly different from reading Chinese, since Chinese calligraphy is symbolic, not phonetic.
Another option is to purchase language tapes or CDs from bookstores. A friend of mine bought the Mandarin Chinese audio course by Living Language recently, but I haven't heard any feedback from him.
Good luck!