Breathing in asanas

i have been doing just asanas for a couple of months now. i havent taken any yoga classes as of yet and i was wondering if there is a certain way you are supposed to breathe while in poses( if you are supposed to hold breath, inhale or exhale a certain way).

i have also read that practicing pranyama can be dangerous without it being taught properley and was wondering how accurate that is. are there any basic breathing/pranyama excersises that i can do without the help of a teacher, that will help me deepen and add to my practice.

thank you