Withdrawals from your Roth IRA are deemed to be done in the following order by law: First, contributions, second, conversions, third, earnings.
In the most probable case, if your net Roth IRA contributions to date are greater than the amount you intend to withdraw, there would be no taxes nor penalties. In this case, the amount withdrawn would also not count against you for ACA MAGI purposes, because you're just withdrawing your own money, so it does not count as income.
If you plan to withdraw enough such that you will exceed all of your Roth contributions to date, then it would depend on if you have any conversions and when those conversions were made. It also might depend on your age, not sure about that.
In any case, you're responsible for reporting the withdrawal on your taxes in Part III of Form 8606. And of course, you're responsible for keeping track of your Roth contributions and conversions so you can properly file taxes in years to come.
Also, you're losing out on tax-free growth on whatever you withdraw over the rest of your lifetime.
Good luck.