How to create text boxes in a post?

I intend on posting a fan adventure on here and on mspfa in a couple days once I get some final details together. Anyways, I ran into a problem. I don't know how to make text boxes on here! Can somebody please enlighten me on how I do such a thing? I am not talking about the spoiler boxes. (Though knowing how to do those would be beneficial as well!) I just would like to make boxes for dialogue in the future for this endeavor. You know similar to the ones in Homestuck? Those dealies.