I recently finished a Taiwanese language* course at Shi-Da (NTNU; 師大). At David on Formosa I wrote a post about Taiwanese language learning materials. It contains some information about text books and other resources for learning Taiwanese.
There are also two pages about the Taiwanese language here at Taiwanderful. I recently updated the learning Taiwanese page. It contains some advice about how to learn Taiwanese and some links to other websites about the Taiwanese language. There is another page about writing Taiwanese using Chinese characters which aims to help overcome the common misunderstanding that all Chinese languages are the same when written in Chinese characters.
* I use the term Taiwanese language (台語). The language is also referred to as Hoklo, Hokkein or Minnan (閩南語).