You can use:
<asp:textbox id="textBox1" style="text-align:center"></asp:textbox>
Or this:
textbox.Style["text-align"] = "center"; //right, left
To center align text
input[type='text'] { text-align:center;}
To center align the textbox in the container that it sits in, apply text-align:center to the container.
Source: Stackoverflow.com