The difficulty of getting started on your writing is so common it has a name “writer's block”.  This is an indication that writing is perhaps an uncommonly difficult task. However, some simple tactics should be sufficient to get past the barriers and begin to write.

No Interruptions
Writing requires an extended period of uninterrupted time; an hour is a good minimum. It is often very difficult to do uninterrupted work in the office. You need to turn off the phones and email, not have people walking by who will say hi, or be faced with other distractions. Very often I find a café a good place to work.  In fact I spend an hour writing in a café every day.

Energy
There is no point trying to write when you are tired. Pick the time of day when you are at your best and write then. For me it's the mornings and if I have something important or difficult I'll often trot off to the café at 7:00 am and, appropriately fueled, work head down for 90 minutes to get a difficult stretch of work done.

Just Do It
Even if you are in a quiet place full of energy you may still find you can't get started. My solution to this is just to start writing whatever comes to mind. I don't care if it's badly worded, full of typos and incoherent. The act of just spilling words onto a page can set you up for a productive session next time.

Typically when I do this I find that the ideas spill out and I get a sense for what I know and what I think. I don't usually edit that word-spill. Instead I start fresh in my next session, but having got my ideas on paper the first time enables me to get past that feeling of not knowing where to start.

Sometimes the word-spill is incoherent and just leads to a second word-spill, not a first draft. It may be that you just need to continue to wrestle with the ideas. It could also mean that the ideas themselves are not coherent. Maybe you had this idea that the Gilligan's Island was in fact a Biblical allegory and decided to write about that. On doing a few word-spills you would probably find that you could not pull together decent arguments. In this case just give it up and be thankful that by “just doing it” you've learned enough to be comfortable abandoning that project.

You have lots of ideas, don't be afraid to throw out the ones that are not going anywhere, and do some new word-spills to get the next idea started.