As a teen, you're noticing your body change. The technical term for this is "puberty." Everyone goes through it and it just means your body is beginning to mature as you grow toward adulthood. The process happens as a result of a long game of Telephone that different parts of your body play to get the changes started.
Everyone is born with a gene called KiSS1 (yes, that's really what it's called!) and another one called GPR54. Your KiSS1 gene produces chemicals called kisspeptins. Your GPR54 does nothing for a long time, until it's time for puberty. One day, your GPR54 gene comes to life and sends a message to your brain that it's time for your body to start changing. Once your brain gets this message, it sends its own message through a hormone called gonadotropin-releasing hormone, also called GnRH. Your brain sends this GnRH to your pituitary gland, which sits just underneath your brain. Once your pituitary gland gets the message from the GnRH, it starts to produce two more hormones, luteinizing hormone (LH) and follicle-stimulating hormone (FSH), to carry the message through your blood stream.
Here is where the paths for the girls and boys begin to diverge a bit. If you're a girl, the LH and FSH travel to your ovaries and let them know they can start producing the female sex hormone called estrogen. The combination of these three hormones trigger your body's changes and prepares it to become pregnant in the future. If you're a boy, the LH and FSH hormones go to your testicles so they can start producing testosterone (the male sex hormone) and sperm. The testosterone is the hormone that triggers most of a boy's changes during puberty and the production of sperm is what now makes it possible to get a girl pregnant.