How far along am I really?

Amy
Hello lovely people, I am so new to this and very curious about just about everything. I'm 99 percent positive that I conceived on May 29th which would make me about 3 weeks, but this app and the doctor goes based on first day of last period which was May 17th. I'm just wondering when I can find out the gender and everything and basically how far along am I? May be stupid apology in advance. Thanks!!