Do you always try to put keywords in your website that other people will search or keywords that you like to search? Like let say you searched the keyword "boom" a lot but others search "baam" a lot, which do you go by? if "baam" and its irrelevant to your website, why do you do that?
its better to go for words which every1 searched for, which u can find out from lots of places. But if a word is irrelevant to your website, its wont be very easy to optimize ur website for the same, so better go for keyword which makes sense wit ur website. For eg: A medical site cannot be optimized for word Java which searched more than any medical word (i assume this)
If your site was new and you want to rank on search engine, it's better to start on targeting the keyword which is not too competitive but not less competitive. Then if you finally rank that keyword you can now target the competitive keyword.
Shoot for long tail keywords until you get familiar with SEO. I used a very competitive keyword to optimize my blog for and now I think that it is not that competitive. The best way to determain if the keyword is competitive is to check first 20 or even more websites that come up for that keyword. Together with a good keyword research tool you can get some decent results. p.l.u.r.