What do I do if someone is using my appleid for imessages

You are sign in here .  .  .  with your Apple ID & Password.

Your Apple ID is the personal account you use with all Apple products and services. It includes the email address and password you use to sign in, as well as the contact, payment, and security details that you use with Apple.

You can’t merge multiple Apple ID accounts.

See -> If you can't connect or sign in to iCloud

If you can't connect or sign in to iCloud - Apple Support

Check your Apple ID via your email box, see this Apple article: If you forgot your Apple ID

If you forgot your Apple ID email address or phone number - Apple Support                  

OR see -> Having trouble signing in?

https://iforgot.apple.com/password/verify/appleid

See this Apple article: If you forgot your Apple ID password

If you forgot your Apple ID password - Apple Support

Then see this Apple article: What to do after you change your Apple ID or password

What to do after you change your Apple ID or password - Apple Support

if someone is using my appleid for imessages

See the following Apple articles:

If you think your Apple ID has been compromised

If you think your Apple ID has been compromised - Apple Support

Manage and use your Apple ID

Manage and use your Apple ID – Apple Support (UK)

See who has access to your device or accounts

See who has access to your device or accounts – Apple Support (UK)

Security and your Apple ID

Security and your Apple ID - Apple Support

Recognize and avoid phishing messages, phony support calls, and other scams

Recognize and avoid phishing messages, phony support calls, and other scams - Apple Support