Your User ID is a special identifier for your account, it is created at the moment you start the game, if you ever need support we will likely ask for it so if you can please include this beforehand.
On the other hand there is also the chance that you are reading this because we asked it as well so... let's get it done!
In the game's Main screen look to the lower left corner, there you should find a button that looks like a cog wheel, that is the Settings button.
^ Take the small cog wheel, circled in blue and tap it
^ And this is what you should get
In this window as you can see above you should get several options, we'll leave them for now
What we want you to focus (for now) is on the bottom of the screen in white, that would be my own user ID however they usually are in this format:
Yourname#XX (where X is a number from 0 to 9)
^ that would be my own User ID
Sometimes the x part can be higher than two digits so look out for that as well.
If you can remember to always include your User ID if you need support.
We hope you find this useful in any case if you do have other questions feel free to ask them.