As mentioned Currywurst is native street food. Another one are "belegte Brötchen" (ready-made bread rolls or half-ones). Nearly all bakeries have this. Kamps or Wiener Feinbäckerei bakeries are a good address. Another special one a soups - Germans like soups in various ways, also Gulasch and Soljanka.
In Berlin you will also find various other street foods, most known is Döner Kebap (German Turkish roots; it is a different one than in Turkey). You will find Falafel, Asian fast food, burgers, donuts, and so on.
And you will also find other very specialised food places. One of my favorites is Melt (Grünberger Strasse) where you only can eat Crepes and Galettes (French kitchen) in an outstanding quality.
Best burger for me is Burgermeister at Schlesisches Tor.
My best Currywurst is at Bier's - it was the owner's name. For Currywurst I would avoid all eateries with a numbe in their name.
Street food trucks you will often find on week markets in Berlin (most Saturdays), e. g. Boxhagener Platz or Winterfeldtplatz.