Overleaf has put together a short How do I use Overleaf help page to give you pointers on exactly that.

You can also watch upcoming and on-demand webinars.

For support, visit the Overleaf Portal or contact the IT Help Desk at [email protected] with OVERLEAF in the subject line.

Here's how to import existing LaTeX documents into Overleaf.

Overleaf supports the packages listed here.