Where can I learn Cantonese online for free ?
No reason why not. You should be able to make a start, at least. Of course Cantonese is different from English – most of the world’s 6,000 odd languages are! Cantonese is just another language and anyone can learn it given the motivation. Do you need a teacher? I think it depends on your own learning style – how much of an independent learner are you?
Whether Cantonese is "hard" or not is relative. The answer depends on your mother tongue and what other languages you already know. If you speak other varieties of Chinese you have an advantage. Similarly, if you speak Asian languages like Vietnamese, Korean or Japanese, you will find that a lot of vocabulary was borrowed from an earlier form of Chinese that was closer to modern Cantonese than modern Mandarin. So obviously anybody with a knowledge of the above languages would have a headstart learning Cantonese in terms of vocabulary.
But let’s assume that you’re a monolingual anglophone, as the other person who answered your question apparently did. The most challenging part of your learning will be the tones. Find a website that gives a clear explanation of Cantonese tones and a way of practicing them.There are few sounds that pose a problem other than some vowels – also found in French and other languages – but not in English. When looking for a website look for one where the materials are based on a linguistic analysis of Cantonese. The pronunciation of vocabulary should be given in an accepted romanization system like Yale or Jyutpin complete with tone marks, not some ad hoc method which is too inconsistent to be a serious learning tool. Ideally, Chinese characters should also be given so that you can ask a Cantonese speaker for help if you want. You will also need sound files to listen to, otherwise it will be impossible to learn to pronounce Cantonese, or to learn to understand the spoken language.
There not much else that is "hard". Learning Cantonese grammar is not difficult, once you get the hang of it. Because Cantonese words are not related to English – except for a few loanwords – you will need to put more effort into learning and remembering new words than if you were learning French or Spanish. But the same is true of learning any other language unrelated to English. The only other thing is that if you also want to learn to read and write Chinese, you will have to spend a lot of time and effort on that part. If you are interested in learning the written language, you also have to realize that there’s a difference in how Cantonese is spoken and written.
It goes without saying that you also need a website that can explain the intricacies of Cantonese in correct and colloquial English.
You can at least get the basics of Cantonese from websites. After all, why pay a teacher USD 20 an hour or so, just to learn a few common words and expressions that are available for free online. Of course you also need somebody to speak to and to correct you. I doubt if many people would be willing to do that for free (I wouldn’t, would you?). Sooner or later you will need to find a private teacher or a regular class where you can get feedback on your learning. To be honest, "free" will only take you so far. Don’t forget that whatever you find online was put together by somebody who used their own time and expertise. Even if that person is not trying to make a lot of money from their website, they still have to pay for their web hosting and other expenses. Don’t be surprised if you find limits on what people are willing to give you for free.
Good luck learning Cantonese.
To find some useful websites, try searching for "learn Cantonese" or "learn Cantonese online" on Google.