So, to use the ubiquitous 'widget' example, if you had a site that was selling widgets you could first try and buy the domain "widgets.com/.org/.net" etc. if this is taken try "sellingwidgets.com" and so on until you get the domain you are happy with.
If your product or service is country specific then I would always recommend obtaining a country specific top level domain (ccTLD – Country Code Top Level Domain) such as.co.uk for the UK, .fr for France, .ca for Canada etc. Most registrars will sell these domains, but if you are struggling to buy one for your specific location type your ccTLD followed by 'domain names' into the search engine and you should get a good choice from there (for example' .co.uk domain names'). If not then feel free to get in touch with me and I will help.
So which domain extension is the best? There are so many extensions these days that opinion is very divided. The official line taken by Google is that the extension does not matter, but all the evidence points to the contrary. In general the top domain extensions in order of preference are
3. .info or.biz
.com's have always been more brand capable and tend to do very well in the search engines.
.org's are good too, but are still often associated with charities and organization.
.info's tend to be a little cheaper but because of this have always been seen as a little spammy. Originally they were associated with information sites, and if you have a site which is based mostly on supplying information then this will still be a good bet.
The use of hyphens in a domain name is another gray area. Usually a hyphen will make your site less brand capable and harder for people to remember, but can still be a good idea if it helps you get your keyword rich domain name. I would not recommend however any more than 1 hyphen. So in our example above you could use selling-widgets.com but I would steer clear of you-selling-widgets.com
And finally we can move on to prefix and suffix. A prefix comes before your keyword and a suffix after it. So for instance in the widgets example your keyword could be prefixed by 'online' or 'your' or 'buy' then making onlinewidgets.com or buywidgets.com
The suffix would simply come after your keyword, such as widgets online.com or widgetsreviews.com
In general I would recommend using prefix and suffix if it is suitable for your business, if you are selling a product online then putting a prefix of 'buy' or 'buying' is very viable. Also of course the prefix and suffix are good if the domain name and extension you are primarily after are not available.
That's it for now. As always feel free to ask any questions or leave a comment.