Authentication docs start here Authentication | Twitch Developers
Then use the oAuth token with Get Users Reference | Twitch Developers to get the user
Sounds like you want to get all channels a user is subscribed to. This is basically not supported. There is Reference | Twitch Developers but you need a pair of ID’s you can’t get “every” a user is subbed to.
Using an oAuth token the represents you (not a user) Reference | Twitch Developers
This information is not in the API
You can get it if/when a user shares their resub via EventSub or PubSub or Chat. So refer to those docs for the relevant events/data