Do I have to tell my boss about my side hustle or side job? Only if it is in your employment contract or handbook policies, that have previously agreed to, would you be required to disclose. 1 in 4 Working Americans make some type of income on the side to make ends meet. That's 50M Americans...it's becoming more and more common and you should feel like you're doing anything wrong.
Can my boss prevent me from having a side hustle? What if there is a conflict of interest with my full-time job? No, however, I don't recommend you start a side business in the same industry, services, products, or try to sell to the customers of the company you are employed with. If you do, yes, your employer could prevent you from doing so, terminate you and it WOULD be a conflict of interest.
Can I get in trouble for not disclosing my side business? Keep it quiet so it doesn't affect promotions or raises, but if your employer finds out...it's ok. Turn it to your advantage! Explain to your employer that it's giving you better management experience, be specific about how it's helping you in your performance in your 'day job' AND also how the extra income has helped you be less stressed so you can enjoy and be less stressed in your daily performance at your 'day job'.
Plus, how do I report the income? Is this going to be a problem come tax time? Actually, it will help you save money come tax time. The tax planning opportunities are incredible! Lots of ways to save taxes. Keep track of your expenses, treat it like a real business, you'll have more success and make better decisions as you are more organized and keep good books. Most of the time you will report this as a sole proprietorship on Schedule C of your 1040...but as you make more and more money you may need to create a formal entity and it also depends on the type of your business.
Anything else to add? Embrace the American Dream! This is your chance to do something you love, take that idea to the next level, turn your hobby into a money making proposition...all the above. Some people say it's too risky to start a small business, I say it's too risky NOT to have something to fall back on. Don't put all your eggs in the corporate basket.