One of the best ways to learn a new language is to surround yourself with it.
So for example of you wanted to learn German, try listening to German music, watching German TV and films etc. Even if it has English subtitles just hearing it can help with your learning.
There's 100s of apps out there that can help you too. Just don't expect to speak it fluently just from learning by a book. 99% of the time you'd need to go to the country that speaks the language you're learning, as there's slang etc that books and non native speakers wouldn't understand