How do I center the labels in my Microsoft excel spreadsheet

Excel has a tool bar (similar to the one on this site when you are answering a question). Toward the right you will see B I U (for bold, italics and underline). Next you will see 3 sets of horizontal lines. This is for lining your text up on the left, center or right. Highlight the text you want centered and click on the middle set of horizontal lines.

In addition, if you right-click in a cell and select FORMAT CELLS > ALIGNMENT, you get a BUNCH of alignment controls... see the circled areas in the picture below.