How can I minify CSS and Java script in WordPress? And is it good for SEO?

you can install WP fastest cache plug in and this will optimize all of your WordPress files.

1. Search for WP Super Minify in the available search box. ...
2. Scroll down until you find the WP Super Minify plugin and click the "Install Now" button and activate the plugin for use.
3. On the left-hand admin panel click on Settings and select the WP Super Minify option.