How to Start Learning Taiwanese
Like any language with difficult pronunciation, it's best to tackle the sounds of the Taiwanese language first. While this can be one of the most daunting parts of Taiwanese, learning vocabulary without knowing how to pronounce it correctly is not a good use of time. It'll end up taking double the amount of time or more.
I personally liked both Aiong Taigi's videos and Bitesize Taiwanese pronounce it like a pro series for getting to know Taiwanese pronunciation better. Although I'd recommend getting a good grasp of the pronunciation and tones (plus the tone changes too), learning Taiwanese pronunciation can be slow going and hard to stick in the mind. Once you've gone through either of those pronunciation courses (preferably both), get learning the basic phrases of the language. Just make sure not to graduate from pronunciation lessons too quickly, while you're learning phrases and vocabulary you'll need to keep coming back to learning the pronunciation rules and nuances unless you are a language genius.
Set your goal
Make sure to decide on your main goal for learning the language early on to keep the motivation going. Languages are learnt by making it a routine, learning a little bit every day if possible. It's easy to get burnt out quickly by studying too hard in the beginning and being unable to keep up that amount of study time as a routine.
Your goal should decide your study method, figure out what you want to learn and make a plan to get there and how you'll do it. Go through some of the free or paid resources listed here and find which ones you like most. Everyone has different learning preferences, so the most popular resources might not necessarily be your favourite. As resources for learning Taiwanese are quite scarce, there isn't a lot of choice, but you've still got a few to choose between.
Listen as much as possible
With Taiwanese pronunciation being quite difficult for most people, I'd advise listening to the language as much as possible, even in the beginning. While you won't catch many words, it gets a good feel for the sounds of Taiwanese. You can do this by listening to podcasts on spotify or watching Taiwanese dramas. If you're someone who gets frustrated over not understanding anything however, you can leave it till you know more basic words. Getting speaking practice with a native speaker as early as possible is also a great way to learn quickly.
Once you're more confident with the pronunciation and have a cluster of phrases at your disposal, it's time to increase your vocabulary. Use this site to practice and learn more words in Taiwanese, or find a flashcard deck on Anki, and make sure to make it a routine