I have created a LaTeX document elsewhere - can I import it into Overleaf?
Yes you can. You can create a new document and copy & paste the text from your existing tex file. If your document contains multiple files, you can zip them up, and upload the .zip file using one of the upload options on your projects dashboard (see picture below).
Alternatively, you can first create a blank project (or create one from a template in the Gallery). When the project editor loads, click on the “Add files” link at the top of the file list, and then select “Upload from… Computer”. You can then select the files you want to upload from your computer, either by first clicking on the "Select files" button to open a file browser, or simply drag-and-drop files from your Explorer/Finder onto the uploader. This animated GIF shows how this can be done:
Can I use git to push a new project?
If you use git, you can also git push your files to your Overleaf project.