I have a website but some times I think that my site need other keywords .Is good finding better keyword or is better let the same keywords and work with them?
You have to used google adword tool for keyword estimation to fine th niche, You have analyse all the possible and go for best one, Some time we are using some blind keyword, I consider them that they are the best, but if we look for trending and keyword search volume estimation tool then we can see that we are far from niche.
One of the first steps in any SEO campaign should be to select your keywords. If you choose the wrong keywords at the start then you can spend ages getting your site to position #1 and then find you've got no traffic because no-one's searching for that term. I'd recommend that you do a full keyword review now - use the Adwords Keyword Tool and Google Insights for Search (http://www.google.com/insights/search/) to determine which keyword is going to drive the most volume for you. Then feel free to change your current keyword to the ones you've selected.
Once ranking the keywords in search engine, try to add other target keywords. This is good to target more keywords.
If you have achieved desired ranking for the present keywords that you are moving on then you need to try for some new keywords which are related to your business and can help to drive traffic on your website so that you can be able to get traffic in from different keywords and different audience.
Its always good to add new keywords and them focus optimization on them as it will help you get high traffic directed to you websites and your new keywords will also have better points in its ranking.