When you are makin' a post or writing a reply, look up in the markdown styling boxes right above the text field; the 3 lines in a list with the downwards facing arrow is the one you want. Second option in the dropdown menu presented when you click on it is "align center". Alternatively, the shortcut is (ctrl +shift + e), and the raw html will be something like div align="center" TEXT /div (properly sandwiched with the appropriate closing tags ofc)
As another shortcut for the latter, when you type and style something, you can also click on the tags at the very end of the styling bar to see "code view", which can... help figure out a lot of simple html. Oftentimes auto-styling is simplistic and prone to inefficiency though, so I wouldn't take the sometimes random stuff it spits out as gospel or anything, hah.