Welcome!

To add a signature to your post, follow these steps:
Hover over the button that says Profile (it's above the green box at the top of the page)
Choose Forum Profile
from there you can modify your signature to show location, allergies, etc.
To modify a post you've made, go to the post and in the top section of that post should be a button that says modify. Make any changes and then click save at the bottom.
In other areas of the board, you post just as you did here. If you don't see a topic you want to post in, you can start a new one by clicking New Topic at the top of the list of topics in that section.