Every day when I come to the office and sit down at my desk I start doing that one thing that I think is most important. It's inevitable that 30 seconds into it, there will be any number of interruptions however. Still, when minor distractions are over, I turn right around and keep working on it. Eventually, when the day is over, I can be satisfied that I did the most important stuff first, despite interruptions.
The alternative is what most people do - arrive without a plan or any idea what to do. If you begin the day unfocused you're like a leaf blowing in the wind. You never know when or where you will end up.