Answer:
You get horny naturally, without doing anything. Our instinct is to be sexual, to carry on the species, so when we see an attractive person, we might start to get horny, that is, to get sexual feelings. Or sometimes we get horny for no apparent reason at all.
Legal and social rules and conditioning usually prevent us doing anything more about this attraction to someone else.
This horniness will usually go away by itself, or by us changing the subject in our minds and thinking of something else. We can also make it go away by masturbating.
You can also deliberately make yourself horny by thinking sexual thoughts, or by stimulating your penis or vagina.
See the related questions below.
Watch some porn on the internet. Play with your body, fantasise about anything you've ever wanted. Strip your clothes off, tease yourself, anything, be creative!