Citations for websites in APA are simple. They consist of four parts: author | date | title | Retrieval/URL

Let's look at each individually.

You have a corporate author (that is an organization, not a person). It is the National Heart, Lung, and Blood Institute which is part of the Nationals Institutes for Health which is part of the United States Department of Health and Human Services.

wew. :)

One school says you should list of all the levels of departments, starting with the largest one first. But, I think, in this case the National Institutes for Health is so well known you could start there. Also, it has a good reputation which immediately gives credibility to your source. So, list the NIH and then the individual institute. Separate them with a period.

National Institutes of Health. National Heart, Lung, and Blood Institute.

Now the date. Easy enough. When you scroll down to the bottom of the page you see it was updated on June 12th, 2015. Your date for your citation would look like this.

(2015, June 12).

Now the title. Easy enough again. Since the name of the disease is capitalized in titles, but not in the text, I would not capitalize it. Since this is an "article" from a larger website, the title is not italicized.

Living with sickle cell disease.

The final part of the citation is the retrieval statement. The URL of your website is preceded by: Retrieved from

Retrieved from http://www.nhlbi.nih.gov/health/health-topics/topics/sca/livingwith

And now we add everything together to get a web-page citation.
 

National Institutes of Health. National Heart, Lung, and Blood Institute. (2015, June 12). Living with sickle cell disease. Retrieved from http://www.nhlbi.nih.gov/health/health-topics/topics/sca/livingwith

The citation should be double-spaced with the first line flush left-margin and all subsequent lines indented.