How much of our DNA is useless junk?

At least 75 percent of the human genome consists of non-functional, junk DNA. The functional portion of the human genome probably constitutes only about 10 to 15 percent of our overall DNA.

Sources

https://www.sciencealert.com/new-evidence-suggests-at-least-75-of-the-human-genome-is-actually-junk-dna