when should I start gaining any weight?

Well I'm 22 weeks and never gain any yet

that's up to you and your OB I'm 31 and still down 24lbs and not gaining

Normally 2nd and 3rd trimester but everybody is different

I gained 18 freaking pounds in my first trimester, and I was diagnosed with hyperemesis and I threw up 95 percent of the day. Everyone is different.

here's a ball park range of how much gain to expect