Trip to Istambul: tipps and infos (Regensburg)

Hello my friends,

I am planning a visit to Istambul with a friens (girl) for next month and I need some advices, informations from locals or people already been there.
What is there a MUST and what can be left out? :)

And about another sensitive topic, is it true that is 'dangerous' to travel to Istambul just because we are 2 girls?!
I am really open minded and I wish sience a long time now to visit the amaizing city, but is there maybe something I should be careful about? Any experience?
I would apreciate every information.

Cheers,
Lavinia