Someone in my contacts is using my number to text other people

if you’re not sending those, then either they’re sharing your Apple Account, or they know your passcode and have access to one or more of your devices, or they’ve cloned your SIM, or they’re spoofing the SMS sending info.

https://help.apple.com/pdf/personal-safety/en_US/personal-safety-user-guide.pdf

Run Safety Check, and verify you recognize all devices associated with your Apple Account.

Enable two-factor authentication if not already enabled.

Change your device passcode(s), and your Apple Account password.

If problems persist, contact your carrier.

I'd recommend changing your Apple Account Password --> Change your Apple Account password - Apple Support

Do you share an Apple ID with someone else?