how can someone hack my xbox 360?

Clarification as to what they are compromising would help. As denis97g states, if someone has your details they can access your account. You can check everything is correct here HTTPS://account.microsoft.com change
password and set up 2 step verification.

Make sure you have a strong password, if you have a simple password they will easily hack you. Put numbers and letters in your password and make a long password and also make strong security question answers...

It's likely that someone got your email and password that you use to log in to your Microsoft accounts/email/skype etc. This would be the same login that they could use for your gamertag on a console. Be very careful online about where you enter your account
info and make sure the site is secure.

It would only be possible if someone else has your email and password.

Download the “Microsoft Authenticator” app for your phone.

This is by far the best 2FA for your Microsoft Account.