I don't think I made any bad mistakes starting out, but I'd advise anyone beginning on Upwork to read the warning about scams. They've increased a lot over the last year. Everyone should read this before they start looking for jobs here:
Scammers pick on new members who haven't yet found work.
Another piece of advice I'd give is to read the forums here, where you'll find advice by experienced freelancers on every aspect of working on this site.
The best way to search the forums is to click on "New Topic", as if you're starting a new thread. As you write the topic in the title box, links to threads on the same subject will appear underneath, and you may find the answer to your question there.