I'm starting to learn Cantonese and I'm not sure where to start. Is it best to start with learning to write/read or learning to speak?
(As a born Cantonese and not a professional linguist)
Cantonese is mostly spoken while the formal written form of Chinese is always 白話文/語體文 ("Written vernacular Chinese", what I managed to find on Wikipedia).
The written form of Cantonese is informal, and mostly used on Internet forum, text messages, modern subtitle, etc. In Hong Kong, the newspaper "Apple Daily" first used Cantonese in quotes in their articles, they were heavily criticized back then, but Cantonese in formal text is ever so slowly getting to appear in formal occasions.
You might notice there are many
口 radicals in Cantonese, they are mostly invented or borrowed characters as those didn't existed in the past and aren't used in other dialects. https://zh.wikipedia.org/wiki/粵語字. Moreover, many of those aren't standardized because they weren't used in writing that much, like those on the Wikipedia page:
伊家 means the same in Cantonese.
In Cantonese spoken regions, "Written vernacular Chinese" is always recognized and written Cantonese is not encouraged for formal texts. One can also read Cantonese from Written vernacular Chinese. So unless you know how to speak Cantonese, learning how to read it might not be your first priority. Therefore, for the best effects and usefulness, I suggest first learning how to speak it.
Written vernacular Chinese:
It depends on what you're trying to achieve. But Cantonese is used more often in speech than in writing: Most written materials in Cantonese-speaking areas still use Putonghua / Mandarin.
Since you commented that you want to learn it mostly to understand speeches in a church, I'd recommend learning to speak first. Learning the characters can be difficult, especially if you don't already read Chinese. You can stick with writing the words down phonetically at the start.
Most learning material about Cantonese will use characters, but I know that there are some "business Cantonese" type books that use phonetic script only.
Learning to speak will not give you any bad habits that would later interfere with learning to read. But if you first learn to read, then you will use your own ways to hear the words in your mind, and it will be wrong (it will sound like your English). You will then have a lot of work to do overcoming those habits before anyone can understand you.
In today's world of digital resources it is easy to find resources for good speech and hearing, and you should use them.
Jave a look of these free materials:
Just curious, may I ask the purpose of learning? Lastly, beware of the tones, there are nine :)