Why do you want to learn a southern dialect?
The only experience I have with Italian dialetti is with Bresciano - a northern language, which is what my husband speaks in addition to Italian. People who grew up 1-2 hours away from him can not understand him when he speaks dialetto, and he can not understand dialetti that come from areas that are not bordering his. So, even though he's from the northern central part of Italy, he does not understand northeastern (Verona, Venezia, Trieste) dialetti, for example. This makes me a little suspicious of the idea that learning one southern dialetto will allow you to understand most of them, but I'm not an expert, maybe it's possible.
His dialetto has a lot of Germanic and French sounds and even though I have very good comprehension of Italian, when he's speaking dialetto, I can neither produce those sounds nor understand what he's saying. So learning dialetto (with some exceptions) is really learning a completely distinct language, made especially difficult by the fact that dialetti are not written languages (in my husband's dialetto, there's no consensus on how many sounds should be spelled, since they don't exist in Italian, so in the few cases where it's written, everyone kind of uses their own judgement on spelling). It's also rare to find people who speak pure dialetto (except for maybe 80+ year olds in the countryside). Even people who use a lot of dialetto tend to mix it up with Italian to different degrees. So finding user-friendly (or any) resources will range from impossible to challenging. I know that the 2 or 3 Netflix original series in Italian feature southern Italian, with some doses of dialetto (I think Napolitano), so you could start there and see what you think.
I ask why you want to learn, because at one point I thought it would be cool if I could learn my husband's dialetto, since some of his family and friends use it, and he said he missed speaking it. He actually told me that even though it bothered him a lot that dialetti were dying out, he didn't want me to learn it, because it felt weird; it's a language that's specific to his little region and in his opinion he prefers that only people who are from there speak it, it's sort of like a badge of honor or a stamp to show where you're from. It really surprised me that he felt like that, because he has zero national/regional pride, and he thinks it's great that I'm learning Italian. If he feels that way about his dialetto, I suspect a lot of other Italians may, too. That doesn't mean you shouldn't learn a dialetto if you want to, just that it will be a very different experience than learning Italian.
Have you checked out this free online course in Italian dialetti from edx.org?
https://www.edx.org/course/dialetti-in-italia-2*I can't bring myself to use "dialect" because the word dialect in English means a regional variation of a language, and each dialetto is a unique regional language that existed long before Italian, and are not mutually intelligible. I'm probably wrong about this, but the English word "dialect" feels like a false friend because I don't think dialetti are actually dialects.
