Help me understand this pls

So I had my second transvaginal ultrasound today which helped the doctor determine I am 6weeks 1 day.! My last AF was ( on dec 28 )irregular so the doctor said it’s irrelevant for calculating how far along I am . With the help of embryo size they determined I might be 6weeks 1day and probable due date would be 9 oct 2018. So ladies help me understand this . When does my first trimester end and second trimester start? I googled the same but it gave me several different answers based on development of fetus , gestation and date of conceiving. I don’t want to go public until I reach me second trimester .. so when would that be exactly be ? I’m sorry if it sounds stupid but I don’t understand a word of all these dates and trimesters 😐

Actually when is the best time to go public ? I want to reveal only when I’m sure the baby is doing absolutely fine since I am diagnosed with PCOS and don’t want to take any chances .

Is 12 weeks the actual first trimester mark?