It depends on what type of writer/person you are. Everyone has different methods.
I sit and write for at least 1 hour every morning. Even if I have no ideas, or it's just nonsense. Sit and write. Get it done. It's an exercise, just like stretching before you go for a big run.
The physical act of writing itself opens many doors of inspiration.
If I already have an idea, write it down immediately. Do not wait for more ideas to come along before you address the first one. Work with what you have, more will come later, usually while you're writing your first idea down.
Start wherever the character tells you to. Worry about a beginning/middle/end later.
if you got a character who has a story already, it's dead easy.
just start on tuesday morning when he pours his first cup of coffee.
and if typing your first sentence itself is what gives you a hard time:
remember you got a backspace key, just in case