Thinking of going fixed for the first time. Anything I should Know

Lots of the above:

1. Use 2 brakes to start with, ditch them later if you want
   
2. Go somewhere quiet for a practice, preferably include some speed bumps, before you go on busy roads.
   
3. Also practise signalling while still pedalling.
   
4. Use foot retention (obvs)
   
5. Think about where you're going to stop at junctions to get your foot position right for pulling away.
   
6. Also get used to filtering on the outside to avoid pedal-strike on kerbs
   
7. If your knees are creaky, start on a fairly short gear (60-65 GI so 42-18 or similar), then gear up if you want more top-end speed.
   
8. As @Hefty said, ignore the feeling on the first few rides that this is a stupid thing to do. It will soon become an absolute joy.