Two techniques for you:
Put your ENTIRE TABLE into an outer table, with one row, and one cell. Set that cell to "align=center":
<html>
<head></head>
<body>
<table>
<tr>
<td align="center">
<!- your inner table HERE ->
</td>
</tr>
</body>
</html>
The second technique is to assign a proper doctype (you need to learn about doctypes), and use CSS. The CSS style, applied to your main containers (like the body tag) would be "margin: 0 auto". This means "0" top margin, and "automatic" left/right margins.