what trimester am I in?????

Tiffany
Ok so I'm a first time mom to be and this whole trimester thing is a bit confusing especially when people ask how far along I am. As of today I am 13 weeks so would that put me at day one of my second trimester or am I still in my first? Please ladies help it's so confusing especially when people ask how many months I am and they look at me crooked like they don't understand. How many weeks equals each month and what tri am I in?