If by “credibility” you want users to trust your website, you may have to do a lot more than write content for some pages. There are a lot of factors that go into a website so that users will trust it. Content is a good start. Make sure it is useful content, well written and free from errors.
You should also make sure the site functions properly and that everything works as it should. Nothing makes users lose faith in a website more than broken features. Also work on making it easy to use. Users also lose faith when the website they are on is hard to use.
Professional design aspects are also very important. If your site looks like it was built by a novice, users won’t give it as much credit.
Here is also a post from SEOMoz that gives a lot of good advice on this topic,