Some basic SEO do's:
Think about your domain name and how information is going to be structured on your website. Use each page on your site to deal with specific aspects of your content. Try not to duplicate content but make use of anchor text links to the relevant page within your site. Visit wikipedia.
Research your web audience. What keywords are people using to find information contained on your website. Are you trying to reach a local audience who know your brand? A national audience who doesn't know who you are but wants a service that you provide? Think about it.
Don't assume that strangers to your website will know what you're trying to say. Spell it out - explain acronyms, describe your process, define your terms. You'll find that keywords will occur naturally when you write your copy like this.
Use meta tags to help categorise each web page clearly for visiting search engine robots and crawlers. The more unique and relevant the meta tags and content are, the more relevant your page will be for searches using those keywords.
Remember that search engines can't see images or videos but they can read filenames and descriptions and tags surrounding such media. Everything on your website should be considered as "indexable" in some way.
Some basic SEO don't's:
Keyword stuffing - why do this? It's ugly, unreadable and unnatural. Would you like to visit a site like this or would you be suspicious?
Overuse of bold text - see above.
Hidden links - this is ugly and suspect to search engines as they see things we don't. Websites can be penalised for shady tactics.
Link schemes - links between relevant sites are good but trying to generate millions of irrelevant links is long term bad. Links to and from your site should help enmesh your website in a relevant context of websites and related information, not background fuzzy noise.
Duplicate content on different domains - generating unique content and web copy is work and "copy and paste" can seem attractive at the time but this can dilute your pages' unique relevance to a subject and can be un-user-friendly if many pages are identical.
Google webmaster tools is worth checking out - remember what Google does shapes the way we use the web - learn what you can from their advice.