I have a few questions as a "starting designer"

I was thinking i wanna try to do this in my free time, potentionally make a living from this since i've always been into the games and i feel like i wanna try this. So i have few questions to start with.

1. What engine should i use and what engine you are using these days?

2. If you were starting from scratch today, how would you learn level design?

3. What are the most common mistakes beginners make in level design?

4. What advice do you wish someone had given you when you first started learning level design?