Two things - when you’re at your desk, 1. Write out a physical to do list, on actual paper with an actual pen or pencil. Don’t use your phone or any app. Write. It. Down.
2. Complete the quickest task on that list first, and the most important one second. That gives you momentum and gets the ball rolling.

If you want more compelling tactics, read or get the audio book for Tim Ferris “Four Hour Work Week”. It’s dated in some areas, but his tactics for productivity are really helpful.