I can't add a text with padding on my blog page.

Hi there,

The alignment problem is causing by an empty paragraph in that first widget.

You can try to delete that or use this CSS:

.inside-right-sidebar .textwidget p {
    margin-bottom: 0;
}

As for inserting the content in blog page, it's an index page so you would have to use GP hooks:
https://docs.generatepress.com/article/hooks-overview/
http://demo.generatepress.com/hook-locations/

With conditional statement:

<?php if ( is_home() ) { ?>
    content for blog page only
<?php } ?>

I trying to use page header to make it look like the about page and contact page on my website but it not working right.

The page header for blog page is in the customizer: https://docs.generatepress.com/article/blog-page-header/

Its still not showing the text on the about page.Example my about page and contact page has a text About and Contact under the navagation.

Got it sorry kinda slow to this website back office! Thanks for responding back so quick!

No problem!