The words used in your website are what makes it relevant for someone searching the internet. If you want your website to appear for certain words then it is essential for you to use these words in the body and in the title tag. If your business is tied to a local area then it is also good practice to use that also.
For example if you are a web designer in Milton keynes the this would make sense for the page title to say "Web Designer in Milton Keynes" also use this for the h1 tag on the home page and try to use these words also in the body.
Using the right keywords is usually pretty easy as well written content will be relvant to your business or subject anyway. Just make sure to use the most important words that you want to rank well for in the h1 heading tag and the page title <title> tag
If you are stuck for keyword ideas then a great place to start is to search the internet for similar subjects and to check out your competition. Good luck.
