Hi there, since I'm not a man I'll leave that for someone else.
One of the first things that happens is growth of hair - on your genitals, underneath your arms, perhaps your chest and more growth of hair on legs and arms.
With girls the same is above (except the chest). Girls may develop breasts right away but most take time to completely grow which is usually around 17/18. The beginning of breast growth looks more like breast buds. They are kind of pointy and most girls are not happy with this and can be sensitive about it. Another process of puberty for a girl is getting her first menstrual period. This usually occurs around the age of 13/14. But some girls have started their period very early, like 9. Other girls do not get their first period until they are 17. When they get their period, they are able to conceive, and must be very safe if engaging in sexual acts. It is unhealthy for such young girls in their teens to be pregnant, physically and emotionally.
Which brings us to awareness of the opposite sex. Sexual feelings emerge and girls begin to notice boys and boys begin to notice girls! That's a fun stage.
A change in emotions is very common to both boys and girls. This is because of hormones fluctuating inside you.
Girls hips begin to grow, and her waist is more obvious.
My brain is a bit foggy so I'll quit now.