Common challenges for English-speakers learning the language
I think the first obstacle most foreigners have to overcome when learning Chinese is the overwhelming feeling you get when you realize what you're getting yourself into - tones, characters, sounds you've never heard before, the seemingly backwards grammar and sentence pattern. Once you determine that you can learn the language because you want to and you will try your best no matter what strange things it throws at you, it all becomes a lot easier. Then you have to tackle each challenge one by one until you've climbed aboard the coveted "Chinese master" vessel.
The first challenge is the strange sounds you hear Chinese shouting at each other on the street. "How does he do that? And how does that other guy understand him?" The sounds can be divided into two categories, pronunciation and tones. The best way to get the pronunciation down is to have a teacher show you the exact pronunciation so you can hear and show you how to place your tongue so you can see it and then keep practicing until you can get it right. There are several sounds that are completely unfamiliar to English-speakers, but with practice and determination, anyone can get it right. Then come the tones, oh boy, "What on earth is the difference between 'ā', 'á', 'ă' and 'à'!?" Again, the key is just to hear and say them again and again until you've got it. Don't worry if it takes a while, it took me 8 years to tell the difference between the second and third tones.
The next thing to tackle is the grammar - the seeming random placement of words to somehow make perfectly understandable sentences. As soon as you think you've got it down, you find that one or two added words totally change the whole meaning. Some people figure that's they just need to think backwards from what they would normally do in English. But there is an order to all this madness and it's actually more structured and constant than English can be. Once you study the basic sentence patterns and get a few conjunction words under your belt, you'll be well on your way to say almost anything you want in Chinese.
Then it's just a matter of expanding your vocabulary, including Chinese idioms and slang, which you can be sure you'll encounter on a daily basis and will make your Chinese suddenly seem so professional. Most of all, have fun! Chinese is spoken by a fifth of the world's population, so get aboard!