Am Start sein is a popular German slang phrase that literally means "to be at the start". It's used colloquially to say that someone is ready, available, or participating in something.
In the song, the singer admires that the person he likes is "immer am Start" (is always ready). He means she's always there and ready to speak up when something happens that she doesn't like. It's a great, modern phrase to add to your vocabulary if you want to sound more like a native speaker!
This song is all about that nervous, exciting moment when you decide to just go for it and ask someone out. The singer, Henning May, lays it all out so simply. He starts by admitting, "Ich weiß gar nicht so genau / Warum ich sie so gerne mag" (I don't really know exactly / Why I like her so much). He just knows he likes her style, her "Art".
What is it about her? He admires that she's always ready to speak up "Wenn jemand was sagt, was / Ihr nicht gefällt" (When someone says something / That she doesn't like). He also thinks she has huge ambitions. The whole song builds around the simple, repeated question: "Würdest du heute mit mir ausgehen?" (Would you go out with me tonight?). He even adds the sweet, slightly worried offer, "Ich würd' dich auch nach Haus bringen / Ich weiß, du musst früh aufstehen" (I'd also take you home / I know you have to get up early). It's a straightforward rock song about liking someone for who they are and taking a chance.