Scandinavian languages are similar in their vocabulary and grammar, but they're not exactly the same. You'd be able to comprehend certain words the same way you understand German from knowing English (lampen, apfel, zucker, etc.), but you wouldn't be able to flawlessly speak the language itself, especially if you start with Swedish. The languages are a bit more closely related than English/German though, and you would be able to understand around 80-90% of Swedish vocabulary and written language by speaking Norwegian. If you want to understand Danish or Swedish as well in addition to your new language, your best bet is probably to start learning Norwegian (Bokmål), as it shares the most in common with the other languages. And Finnish is just completely different.