You will get an official offer through Upwork which you then accept. Do not start working until the contract shows on the "My Jobs" tab and the client's payment method is verified. If it's fixed rate, be sure the milestones are funded.
when they send you the offer it tells you in the message room how much is funded. Always kick back half funded milestones and tell them that they need to fund the whole thing so you don't wind up like half of the people on this forum who don't get paid cuz they accepted a half funded milestone.