Use hashtags and then a space for bold.

One Hashtag followed by a space looks like this.

Two like this

Three like this

Four like this

TO center, put a "< center >" before you text (But take the spaces out)

And put a "</ center>" after your text (But take the space out.)

Or combine it with the BOLD HASHTAG Trick to make it look like this

Give it a try in a reply here and see what you can do.