Welcome to the forum.
I started my new job about 3-4 weeks ago. Prior to that I haven’t had a real job in about 17 years.
The more you think about it the worse the anxiety becomes. If you didn’t have this new job you would feel fine right now so it’s all in your head.
On my first day I just concentrated on the tasks at hand. I didn’t worry about anything else, or what the rest of the day would hold. Just baby steps going from moment to moment.
Don’t get lost in your anxious thoughts.
Edit: So if your doing something and get anxious, just tell yourself, right now I am just photocopying some paper, there is nothing to get anxious about that. Then do the same with the next task… whatever your job entails. Ground yourself in what you are currently doing and don’t get lost in thought about scary thoughts that probably won’t even come true.