Not many people have tested their own theories over a long period of time to see what works and what doesn't. All the advice you're about to read is from first hand experience. I worked all this out with a lot of trial and error. Before you touch anything, you have to decide which words are best suited to the topic of your site. Think carefully because the key is to match the words your potential visitors are likely to type into a search engine.
Users tend to be fairly general with their initial searches, then they narrow it down.
So, if you have a pet shop the keyword 'pets' is going to be more powerful than 'dogs'. You can choose between a short, powerful keyword list which is likely to put you high up but with only a limited number of words, or you can have a long list which will put you slightly lower, but with a broad spectrum of keywords. And Descriptions Describe your Web site, not your company. The name of the game is getting Web site visitors. Once they've arrived you can talk to your heart's content about your annual turnover or whatever.