If you are an institutional user there is no need to sign up, click here for more details.
Click on the Sign up button, which can be found in the top right corner on the Figshare landing page.
You will need to enter a few details and accept the Terms & Conditions. From then on you will just need to use the Log in button.
To start uploading your data click on My data located on the top left.
Please note that once you publish an item, your profile page will then become public. For more information about your profile page and tips on how to maximise your discoverability please read https://knowledge.figshare.com/articles/item/maximising-your-research-profile