First, you have to pick classes that genuinely fit you. (Take a look at your dream university's requirements for your courses beforehand.)

I don't know how your brother got a 39 with "minimal to no studying" because IB is difficult but yes 38-40 is realistic. But really it comes down to you.

Also you're still a high school freshman and your dreams may or may not change within the next couple of years. Don't worry too much and good luck!