To edit Markdown and LaTeX, just create an account and click the "New pad" button on your profile page.
To execute code, you will need a sandbox.
All sandboxes run on powerful Amazon EC2 servers. The smaller sandboxes are good for working with memory-efficient languages or non-demanding tasks. Larger sandboxes are best for intensive work.
Your first sandbox comes with a 7-day free trial. All of your sandboxes will appear on your profile page.