What do I do if someone else is using my iCloud address?

If you see no charges it may be phishing. Scams (e-mail, text messages, and phone calls) are getting very good at closely imitating true Apple communications. Always be cautious. Here are some guidelines:

Identifying legitimate emails from the iTunes Store - http://support.apple.com/HT201679

Recognize and avoid phishing messages, phony support calls, and other scams - https://support.apple.com/HT204759

If you are uncertain about a message and a resource provided in that message, do not click on any links. Try to independently verify the resource by going to a support page on apple.com and use that to contact Apple or to verify the service the message is telling you to use. e.g. See your purchase history for the App Store, iTunes Store, and more - Apple Support

Forward email attempts as an attachment (in MacOS Mail use the paperclip icon) to: reportphishing@apple.com then delete it.

Here's more, some of which is not in Apple's documents but probably has more specifics for identifying bogus messages. I'll post it separately since it is not part of their official guidelines.

- Apple e-mails address you by your real name, not something like "Dear Customer", "Dear Client", or an e-mail address*.

- Apple e-mails originate from @apple.com or @itunes.com but it is possible to spoof a sender address. "Apple email related to your Apple ID account always comes from appleid@id.apple.com." - https://support.apple.com/HT201356

- Set your email to display Show Headers or Show Original to view Received From. Apple emails originate from IP addresses starting with "17.".

- Mouse-over links to see if they direct to real Apple web sites. Do not click on them as this just tells the spammer they have a working e-mail address in their database. If you are unsure, contact Apple using a link from the Apple.com web site, not one in an email.

- Phishing emails may include account suspension or similar threats in order to panic you into clicking on a link without thinking. They may report a fake purchase in order to infuriate you into rashly clicking on a false link to report a problem. Mar 2018 post by Niel https://discussions.apple.com/message/33129140#message33129140 - "Emails saying that your Apple ID has been locked or disabled are always phishing. If one actually gets disabled, its owner will be told when they try logging into it instead of through email."

- Apple will not ask for personal information in an e-mail and never for a social security number.

- Scams may have bad grammar or spelling mistakes.

- Apple will not phone you unless it is a response to a request from you to have them call you.

* Exception: https://discussions.apple.com/message/33701414#message33701414

Forward email attempts as an attachment (in MacOS Mail use the paperclip icon) to: reportphishing@apple.com then delete it.

If this is with regard to a supposed purchase, this Apple article has relevant information and web links for checking if you really have made a purchase or paid for a subscription: If you see ‘itunes.com/bill‘ or an unfamiliar charge on your bank, credit card, or debit statement - Purchases made under Family Sharing might be charged to the organizer's card but will not appear under the organizer's purchase history or subscriptions. Ask family members about those or check your receipts. --> https://support.apple.com/HT201382 Apple will email a receipt to the Family Organizer if a purchase is made on a card held by the Family Organizer. This will have the Apple ID of the purchaser, which you should recognize, but won't have specific about what was purchased.

Read this link "If you think your Apple ID has been compromised" --> https://support.apple.com/HT204145

Also make sure you are using "Two-factor authentication for Apple ID" --> https://support.apple.com/HT204915

Using it how?

https://support.apple.com/en-us/HT204145

Sounds like someone made a typo when they set up those accounts. You can tell those companies, but otherwise just ignore and delete them.

Ok, then it is most likely phishing emails pretending to be legitimate companies. You’ll want to just delete them and don’t click any links in the emails or reply to the emails.