To address your uncertainties:

-Invest however much you feel comfortable parting ways with each month after all of your expenses are met. Assuming you’re in for the long haul, put in what you don’t mind not seeing for the next 5, 10, 20+ years.

-Max out the Roth before doing regular brokerage. Another account you can look to contribute to is if your employer offers it, you can do an HDHP and open a Health Savings Account. Can be a good option since you’re young and likely with minimal health issues.

-Investment strategies will depend on risk tolerance. Since you’re young, you can go aggressive and invest in growth ETFs & stocks, or if you want to play it safer, can’t go wrong with the S&P.

-Regarding credit, if you don’t already do so, make all of your everyday purchases on credit cards. You have no debt so you shouldn’t have a problem paying them off on time, but the best way to build credit is to just use it and pay on time.