How interesting that we all have such different first symptoms!
My first symptom of pregnancy is that my waist expands *immediately* (like within 5-10dpo), so all of my pants are tight. This has happened with both pregnancies. With my first, my parents came over for lunch, and I was wearing sweatpants instead of jeans. I was 9dpo. My mom told my dad on the way home that she thought I was pregnant and that "the waist is the first thing to go!". She was right!
I didn't test until a few days later. That preg, I was in maternity pants (after outgrowing the rubberband trick) by 8 weeks. This preg, I was in maternity pants by 5 weeks. I'm sure if there's a third, I'll be in maternity like 5 minutes after conception takes place.
The other symptom I get fairly early is slight nausea. I usually get it around 7-8dpo. I can eat normally and such, but I just feel a tad bit of nausea. Of course, that symptom comes when I have sinus drainage too, so the expanding waistline is the better indicator of pregnancy for me.