Does a single college student making over $12,000 pay taxes on all of the income or each additional dollar over $12,000?

Filing as a single college student in 2018 my income was under $12,000 and untaxed. If I make more than $12,000 this year will all of my income be taxed or just each additional dollar over $12,000? For example if I make $15,000 will the $15,000 be taxed or just the $3,000?

Thanks