Go to:
appleid.apple.com
Login with your user ID and password. Hopefully, whoever is in your account hasn't changed the password. If they have, you'll have to try and gain back control.
If you can get in, first check to see what devices are attached to your account. Click on the Devices link. If you see any you don't recognize, click on the device and then the button to Remove from account.
Click on Sign-In and Security, then the Password button. Change the password to something difficult for anyone else to guess.
Click on the Account Security button and enable two-factor ID, if it isn't already. Make sure to check and see there are no trusted phone numbers besides yours. Once enable, anyone who still manages to guess your password will not be able to get into your account without the 2 factor ID notification. You'll know someone tried because you'll get a notice at your trusted phone number. Deny access to any request you didn't initiate.