If you're not using a beta or mod, do this:
1. Make a save.
2. Press "[untitled simulation]".
3. Enter a name and description.
4. Check the little box that says "Publish?"
5. Save simulation.

You have to open TPT(main version only, doesn't publish if using a mod/beta) and click the box on the bottom of the screen where it says "Sign In." Then, make a save or load one you already saved. Then, click the bottom box where it says "Untitled Simulation." A screen will pop up with everything you need to publish the save. Also, you can make tags by clicking where it says "no tags set."

@photonstream
Your'e welcome. If you have any questions, you can always ask someone here(as long as you check the search bar to make sure nobody ever answered the same question before, people get pretty grouchy about duplicate questions here sometimes!)