This is really just an html question, not specific to WordPress. Look into checked="checked" (for check boxes) or selected="selected" (for selects, radio buttons, etc.) In your case,
<input type="checkbox" name="showS" value="true"
<?php if (get_option('showS')==true) echo 'checked="checked" '; ?>>
Since this is WordPress, though, I should also be reminding you to use the Settings API where possible. Don’t try to sanitize and validate all of the inputs yourself unless you really know what you’re doing. Here’s a link to a tutorial on the Settings API: