Nofollow for internal links

I have two questions:

1- If we use internal link from page A to page B, Will the credibility of the page A be reduced?

2- Should we use Nofollow for internal links or Do follow?

1. No
2. You really need to read more about "nofollow" attribute. Google: "Use the nofollow value when other values don't apply, and you'd rather Google not associate your site with, or crawl the linked page from, your site.". So you're saying your internal pages are so bad you can't vouch for them?

Keep your internal links d0follow so the link juice can naturally flow through your site.