SexualArousal
Sexual arousal, or getting "turned on", is when your body gets physically excited and prepares for sex. Several physical and mental things happen when this occurs.
You usually get sexually aroused when you think something sexual you'll like might happen. You have to have hit puberty to get sexually aroused.
Sexual arousal in boys
Boys can get sexually aroused even just looking at girls, or having sexual thoughts. It only takes a short time for boys to be aroused, seconds even.
When boys get sexually aroused they can get erections?, and their faces can flush. Those are the physical changes. Boys will also desire to do something sexual when aroused.
As a boy, you might want to avoid unwanted arousal, the only advice I have is to try not to think abut sex, and to turn away if something sexual is happening. If you're like me, you don't want to get aroused unless you can actually have sex, the only advice I have is to avoid sex and not to think about it.
Sexual arousal in girls
Girls tend get sexually aroused when something sexual is happening, though they can get aroused for other reasons. It usually takes time for girls to get aroused, like 10-20 minutes.
When girls get aroused they get lubrication in their vaginas, their clitorises get engorged a bit and come out from under the clitoral hood. Also their breasts swell temporarily and their faces flush. Being aroused also tends to make girls want to do some sexual activity.