Use the White House contact form: https://www.whitehouse.gov/contact/
Send a letter to the White House mailing address:
The White House
1600 Pennsylvania Avenue NW
Washington, DC 20500
Use the official White House phone line: 202-456-1111
Use the official White House switchboard: 202-456-1414
If you mean Donald Trump specifically, use his official website contact page: https://www.donaldjtrump.com/
If you mean a campaign or political organization, use the contact information listed on his official website
Do not rely on unofficial email addresses or social media messages as official contact methods