If its your first time, don't worry, some blood will come out and onto your underwear and I'm not sure if it's the same for everyone but when I first got my period, the blood was brown, just indicating that it was old blood though.
Sometimes I get hot flashes mixed with the chills and feel really sick. One time it made me throw up. And during my period, its really painful, it kinda feels like my area is tied up in knots. Another way to tell is if you think you feel 'warning signs', you can go to a bathroom or your bedroom and, with a mirror, you can look at your area. The lips will be hidden behind the skin where the pubic hair grows. I learned this from a friend and I'm pretty sure that there is a medical video about it on YouTube.