Hi Mario, 

You've not given us any model or engine or age details about your car so the answer is going to have to be quite generalised. Cold-starting issues with diesels often boil down to glow plug problems, whether they need replacing or they're not getting power. That would be the first thing to check. That doesn't account for the fuel leak you've described, though and if that persists I'd suggest you get the car to a good mechanic as soon as you can.