re: "I've never used Upwork. I am looking for a person who knows how to set up a Youtube channel. They all list themselves as Youtube marketers. How can I trust that they have any experince in what I want to do?"
In order to find somebody to set up a YouTube channel, you need to state exactly that in your job posting. Be concise, but be clear about what you need.
Post a public job posting with that information and you will be sure to have people apply who can help you with what you need.
re: "How can I trust that they have any experience in what I want to do?"
There is no way to know for sure if a freelancer has sufficient experience to help you with what you want to do. You can read their profile pages. Often these are 100% accurate. But not always.
My sincere advice to you is this: DO NOT TRY to pick the perfect freelancer, or even the best candidate, from among those who apply to your job posting.
If this is an important project to you, then hire multiple people. Hire 5 or 6 who you think might be able to help you. Then spend some time just talking with them for a while. Ten minutes. Or 30 minutes. Or an hour if your time with them is particularly illuminating and helpful. Do this after you have hired them. Pay them for their time as consultants. You can do this by phone, Skype, chat, email, Zoom... any method you prefer.
Then continue working with the one person (or multiple people) who you think will provide you with the best help for your project.
Here is a big mistake that many "rookie" Upwork clients make:
They hire only one freelancer.
If you do that, then you have no way of comparing that freelancer to somebody else.
What if you hire only ONE person and setting up a YouTube channel is VERY DIFFICULT.
But if you had talked to a couple of other people, you would have learned that it can be very EASY...