Wikipedia:Writing better articles and Article development are excellent references.

Some pointers, starting with the most important four, are:

  1. Notability The subject of the article must be notable. They should have significant coverage from multiple reliable sources.
  2. Reliable Sources - Reliable sources must be used, generally meaning published works that have gone through an editorial process.
    1. Books are great, unless self-published, which means that they have not gone through a formal editorial process Newer books are better than older books, if there's a choice, because the information is may contain modern understandings and conclusions by noted historians of previous publications. Magazine and newspaper articles are also great - because they have gone through an editorial process. To check websites, or online media, check the "About" or other areas to see if there is an editorial process and/or the writers would be considered experts, e.g., historians with published books, etc. Side note: Genealogy books can be problematic and should generally be avoided unless from a respected source, like Burke's Peerage.
    2. Sources that should not be used include user-generated content, such as that comes from personal websites or sites that take contributions from users like imdb.com and Find a Grave. Personal genealogy websites are particularly problematic and prone to error and should never be used.
  3. Paraphrasing - Content should be paraphrased. See close paraphrasing about how to paraphrase content. I also posted a message here with examples in this diff.
  4. Verifiable - Sources should be WP:Verifiable, which means that google book diffs are not considered reliable sources, because not all of the content can be viewed within context to ascertain the full meaning of the information. Providing links to sources is very helpful with verifiability. Formatting citations properly helps with verifiability and so that readers have a better understanding of the sources.
  5. Citation format - There are various citation formats, but the best way to ensure that citations are properly formatted are to use citation templates. You can also use tools, like ProveIt, that will create citation templates (I find it a little cludgey though for editing citations). Reftag is a tool to format google books, but the user should double-check that key information is complete - particularly with older books it seems.
  6. Citation content - Citations should contain basic information: title, publisher, date. If the author is known, the author. Applicable urls with the accessdate should be included.
  7. Sections - Sections help break-up and organize the content.
  8. Section headings - Just capitalize the first letter of the heading, unless it is a proper noun. So, "Early life" rather than "Early Life".
  9. Content - Article content should be encyclopedic. See What Wikipedia is not. It should not contain genealogical content or extraneous detail about someone's life.
  10. Encyclopedic style - Content should be written in an Encyclopedic style, concepts including summarization and brevity.
  11. Punctuation goes before the citations, per WP:PAIC.
  12. Citation order Kind of nit-picky, but if there are multiple citations for a sentence or group of sentences, they should be in numerical order ([5][17][21], not [17][5][21])
  13. Content covered by citations - It is assumed that all the information between one citation or set of citations and the next citation is covered by the ending citation. Every time that there is new content, there should be a source. If there is no reliable source for the information, it should not be included, or it would be considered WP:Original research.