The easiest way is to use a css background image for your logo.
Go to line 65 of default.css and change
#blog-title {
font-family:Arial,sans-serif;
font-size:34px;
font-weight:bold;
line-height:40px;
}
To This:
#blog-title {
background: url(images/path_to_your_logo.png) no-repeat;
display:block;
width: 00px; /* enter logo width */
height: 00px; /* enter logo height */
text-indent: -999em; /* replaces blog title with your logo */
}