Do you know a good app to learn Persian? If I learn Arabic gonna be easy to learn Persian? What do you think?
Knowing Arabic will help you a lot for vocabulary, but there are many many false friends( words that are the same but have a different meaning in those languages). If your goal is Persian, I'd say there is no need to learn Arabic, but if you like Arabic, keep going!
I'm doing an unofficial Persian Course here, feel free to check it out.
I've also heard many people talking about Easy Persian, I think it's pretty good.
Of course, there are courses on memrise, which has an app too. I never used memrise so I'm not sure.
There's Rosetta Stone as well, but it's not free.
Hope you'll finally find the best method for learning our language and enjoy! :)