Someone please help..template ?

Hi. I have my site up and want to change wp theme to my sites them. I realize we have to upload styles.css , index.php, wp-layout.css, and comments.php to the themese folder in wp1.5.

However, I dont know anything about styles.css etc and was wondering what I need or dont need with least effort in styles.css to not mess up the look and feel. All I want in a regular white table is the basic wp-blog info neseccary to keep the blog. I dont want it to mess up tables sizes, etc on my site.

What do I need to do? Please be as specific as possible, I m a total newbie to styles.css.

Many thanks in advance.