Go to the "Find Work" tab, and apply to positions you feel you would be good at. You have a limited number of bids you can propose every month, so make sure you're applying for positions that: a) you actually want, and b) that you have a chance of succeeding with.
You'll want to ensure that you have a top-notch profile first, as it will be included with your bids. I'm still fine-tuning my profile myself, but there are many veteran freelancers who are willing/able to offer advice to make sure your profile is adequate. Don't neglect your portfolio and your past work history - these things are a great service to you. I had zero offers before I started working on these, and then I got two positions that I love in the same day after making the changes.
If you can look through here and find the link to the Upwork webinar (or if you're enrolled for the next one) you can get a wealth of knowledge and insight to help you on your way.
And as for payment method - the clients will pay you, typically after the work is done and verified. Their payment will stay in a review period for a few days to make sure everything's in order, and then you can begin requesting withdrawals once you've built up some money.
Big advice, though -- find the webinar. The webinar is your friend (within reason). It'll take a bit of cultivating to get you to where you need to be, but it's overall a rewarding experience learning how to define and market yourself. Welcome to Upwork!
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