imo Duolingo is pretty good for learning any language. I've only used it for Italian though. People say Swedish is harder to learn than English, but that probably because they are used to English grammar. Norwegian and Swedish grammar is pretty similar afaik. You'll probably mostly have to learn the new words and the correct pronounciations.
Non-nordics have a tough time with how we pronounce a few of our letters, as well as å ä and ö. For example the "sk"-sound is tough for them. Englishmen just starting off learning Swedish also substitute "the" with "den", which is only correct sometimes. There is no direct translation of "the" in Swedish, it's instead substituted with a "-en" or "-et" suffix. For example, "the pear" would be "päronet", and "the car" would be "bilen". A lot of non-Swedes would probably be inclined to say "det päron" or "den bil" instead.