Someone said they would boot me offline I need help

If it ever does happen, have a look
here.

You can contact your ISP as well.

Ddos someone is against the law, and your ISP may take legal action against them.

I would also block this person , so they can no longer see or chat and send messages to you. 👍

Microsoft or the Xbox Enforcement Team cannot do anything about any harassment you encounter online unless you fill out an in-profile report on Xbox Live, as previously mentioned. You should fill it out to the best of your abilities with option of what kind
of report it is, as well as explaining why with the characters given in the report menu.

It is wrong and never a good idea to oust someone online publicly regardless of what they said to you, always use the report and block system available on Xbox Live and the problem will be dealt with.

There are also Microsoft provided resources about what to do when threatened with a DDoS attack:

https://support.xbox.com/en-US/xbox-one/networking/dos-attacks-faq

Fortunately, when you file a complaint with microsoft, they’ll be able to see that they said this to you as well as harassed you and you should have no problem getting them banned