How does one get a scalp fungal infection?

Humans. Animals. Soil.

How does one acquire a scalp fungal infection?

Here are a few pointers about scalp fungal infections. Fungal infections of the scalp are broadly known as "tinea capitis." These types of infections are a lot more common in children than in adults. In fact, when I identify tinea capitis in adults I'm often a bit concerned about the overall state of that patient's immune system or some other reason that would predispose them to a fungal infection. Tinea capitis is rare in adults. 
A number of different types of fungi can cause tinea capitis. They are typically caused by fungi called dermatophytes rather than other fungi that can invade the body. An individual can acquire tinea capitis from another person (and these fungal organisms are known as "anthrophilic") or from an animal such as a dog, cat or horse (these fungal organisms are called "zoophilic"). Rarely an individual can acquire tinea capitis from fungi living in the soil (and these are typically from fungi known as "geophilic" fungi). Tinea capitis acquired from the soil is not very common. Organisms such as Microsporum gypseum and Microsporum fulvum are the more common causes of geophilic tinea capitis.