Go here - https://adwords.google.com/select/KeywordToolExternal Click the button for 'Website content', add your url in the box , click 'Get keyword ideas'
You should also consider what your audience will be looking for, never just assume. Its worth asking someone what they would use to find your site or what phrase they may enter into google to find a site like yours. Use that to start your search and the flood gates will open up.............. Don't forget the longtail keywords can be worth fortune!! Cheers
If you're just starting on this, use Google AdWords and Google Trends. By the way, do you have a built website already or you're still just planning to build one?
try to separate your domain name for keywords e.g. getitdone.com keywords = get, it, done it can be reffer to Information Technology
Google adwords is useless if your site does not contain good content. I would rather suggest to check the most generic keywords of your website. what service your site is providing OR what are your products or product line? Get some idea from these things and list down at least 10 most relevant keywords. Then use them as primary keywords and search for their variations in keyword suggestion tools. Pick up the most relevant keywords depending upon the competition and search volume and target them in your website.