Another centred header question

Hi there,
Can you please help me:

• Make the header fixed without shrinking
• Centre the logo
• Centre the menu underneath the logo

I’ve used this for preventing the header from shrinking:

#header_main .container, .main_menu ul:first-child > li a {

height: 88px !important;

line-height: 88px !important;

}

But various other quick codes I’ve used for centring haven’t really worked properly.

Thanks!