There's really no right or wrong answer here. From my experience, if it's your first time freelancing, the hourly protection system in upwork helps you and the client in ensuring work gets done.
If you are already doing freelancing and simply want to try out upwork, then by all means, feel free to charge the way you normally do.
Another factor to consider is the industry. Personally, the creative industry (writing, graphics, etc.) are the ones where clients usually set ridiculously low prices. But there are jobs where hourly rates averages $50 and up, which is way higher than the minimum wage is pretty much all countries.
ONe way is to go for a mix of fixed and hourly rates. You just have to be willing to spend time to educate and communicate clients on the value you bring to the table. (hint: it's about the results, not the experience. e.g. grow traffic and leads vs writing blog posts for 5 years).