Use the alignment options in the Paragraph palette.
Here's the relevant excerpt from After Effects 6.5 Help:
"Aligning and justifying text
You can align text to one edge of a paragraph (left, center, or right for horizontal text; top, center, or bottom for vertical text) and justify text to both edges of a paragraph. Alignment options are available for both point text and paragraph text; justification options are available only for paragraph text.
To specify alignment: In the Paragraph palette, click an alignment option..."
I dont recall if there is a Grid in 6.5..but that aids one centering text in the
frame (Comp)..if that is what you are asking.