Please do not work on any project until a client has sent you an offer, you have accepted it, it's showing on your My Jobs page as an active contract, and the client's payment method is verified.
The client clicks a button labelled "Hire".
This sends a message to you and also causes an "offer" to show up on your Upwork site.
When you look at the offer on your side, you see a green "Accept Offer" button.
After you click that button, you will see the job listed here:
Upwork -> My Jobs -> My Jobs
Then you can start working.