Just started 1st time period. Any Advice/Help?

(First of all, I'm 12, and I am on here cuz I wanted to find something that could help me out thank you)

It all started when I went to my friends house around 11:30 & today was conference week so we had Wednesday off. It was lightly raining for the last couple of days and we thought why not go into the jacouise and pool. After we swam for about 2 hours, we got out, dried up but didn't take shower (cuz we were lazy), then I went to grab my underwear and then bam, blood that looked like I pooed! I went up to my friend and said "I FREAKING POOED!"We start laughing then run to her mom to make sure cuz my friend was uneasy about it. She laughed, thought it was fake (drew a lot of marker on it?), then she said she thinks I started my period. I check my bathing suit bottoms and I had a 50cent coin sized little light blood spot, I started crying cuz I've always been my dad's little girl cuz my mom was out of my life and we always teased my sister 1 yr younger than me cuz she started her period before I did. I would really like some help and advice and hopefully your stories. Thank You! <3