1) Make a list of the service pages that you want to have on your site (implants, dentures, root canals, etc).
2) Sit down and just write. And don't worry about keywords, etc. at this point. Just write what comes naturally.
3) If it starts to get past 250-300 words, break it up into two pages. For example, if you start to write about implants and you get into bone grafts, it makes sense to make bone grafts a separate page.
3) Regarding SEO, you want to format the pages in a specific way -- title tags, heading tags, etc.. The video below talks about how to structure your blog posts and pages for SEO.
4) As for keywords, keyword research is an entirely separate subject. The fastest way is to head to SEMRush.com and type in keywords and see which ones have the highest search volume. The other way to do it is to look at Google AdWord's keyword tool (not 100% accurate b/c this is PPC data and not organic) -- but it's close enough.
5) Last resort -- use stock content as a baseline and customize it.
Ultimately, you are 100% correct to write you own content - the best content ALWAYS comes from you!