It is always best to book a domain name which comprise the main keyword or get close to it. However you must understand that search engines are looking more into the content than the domain name or in other words just because you have your main keyword as the domain name you are not ranked high.

However they are perceived to be giving a certain weight to the keyword domain name relationship. However what you must understand is that good keyword domain name relationship is no match for rich content.

What TLD (.com, .net, or?) to choose? 

This can be a tough question. It depends upon you! It must ideally depend upon the kind of site you are trying to build. For all general purposes sites with some business interest’s .com is the preferred TLD. .com has the advantage of being the most booked after TLD. Even non commercial sites are often seen booked as .com’s.

However choosing the first part of the domain name (before .com) may be much more critical than the TLD itself. This is since your site is primarily going to be known for the first part and not for the TLD it associates with.

Content is more important.

What we have seen is that, after all, how content rich your site is, how well structured it is, minor things like proper titling; are the things that matters as far as search engines are concerned. After all the most important thing you must be looking for is coming on top in Google!

Hence don’t concern too much with what the TLD can be but care not to go for any yet unheard of TLD as it is always better to tell .com or .net or .org to someone. They are definitely the most well known TLDs presently.
 
Next: Things to care