Locking a side bar

You can do that by changing the CSS code, but you really should not be making changes to any theme files — as is, any changes will be overwritten when WP is updated. Use a child theme or custom CSS plug-in to make changes.
http://codex.ww.wp.xz.cn/Child_Themes

Once you’ve set that up, add this to the CSS:

#page {
   width: 1000px;
}