If you want to center (or align left or align right) the post title, sign into Dashboard > DESIGN > EDIT HTML to open the template editor and search for .post h3 (use ctrl+F) or code that may look something like those below (code below for Minima template, yours may be slightly different):

Reminder: Always backup template and backup gadgets before editing the template.

.post h3 {
margin:.25em 0 0;
padding:0 0 4px;
font-size:140%;
font-weight:normal;
line-height:1.4em;
color:$titlecolor;
}


The just add this line text-align:center;

.post h3 {
text-align:center;
margin:.25em 0 0;
padding:0 0 4px;
font-size:140%;
font-weight:normal;
line-height:1.4em;
color:$titlecolor;
}


preview and if OK, save template.

If you want to align the post title to the left or to the right, just substitute text-align:center; with either text-align:left; or text-align:right;