When do you become professional?
You can certainly write without any desire to become professional. I think in every writer’s heart however is a desire to become professional, to see their name in print. Some writers may feel intimidated to send their work out. They may suffer from imposter syndrome. Maybe they feel they’re not good enough. That’s not up to you to decide. Get over the fear. You won’t know how anyone feels about your writing unless you start marketing your work. Follow Robert Heinlein's Rules of Writing:
i. You must write.
ii. You must finish what you write.
iii. You must refrain from rewriting, except to editorial order.
iv. You must put the work on the market.
v. You must keep the work on the market until it is sold.
Pay close attention to the last two rules, because if you have ANY desire to cross the threshold to professional, those two rules are imperative. Lose your fear and start marketing your work. I’ll add a sixth rule. One I’ve lived by:
vi. Don’t let anyone tell you No you can’t do that.
So, to answer question #2, you become professional when someone pays you in money for your writing. Note I said money, because many small press markets pay in copies. In other words, you will receive copies of the magazine, newspaper etc. where your story appears. Until you receive monetary compensation you are not considered professional. Continue marketing until you receive monetary compensation and even after your first payday don’t stop.