Generate decent content that relates to those keywords, optimize them for search engines, get quality relevant backlinks and your website ranking will improve.

Gather some quality backlinks from submitting your website on articles directories, Social bookmarking, forum posting.

Cheers,

Or if you have the money, you could hire someone here on the warrior for hire section to do some backlink building for you. If you choose that route, find someone who does Tiered linking, it's more effective.

If you want to do it on your own... post about 10-20 links, and then post about 10-20 links to each one of those sites. That generally gives a decent boost.

Then post another 10-20 on different sites and backlink those.

Keep backlinking til you're ranked.

Write content on those keywords and start posting them on High PR Article sites like ezine, goarticles etc. Do Social bookmarking and participate in forum.

The old saying "content is king" is very true. Create some content with those keywords being the centrepiece. Get some quality backlinks too and then you'll be sitting pretty.

you can do,
directory submission
social bookmarking
article submission
link buiding
blog creation and commenting
this will help to improve your keywords ranking

I have some keywords which have low ranking and I want to increase my keywords ranking but I don't know how to do this.

Yup yup...."content is king"

Add the unique content. Do proper on page and off page optimization of your site and you will surely see the change in rankings.

As rank for website shows the reputation of website. And as per new algorithm , search engine consider the quality content first for assigning the rank. So provide best and unique information on your site with many techniques , and get more traffic and also try to get quality backlinks. And this way you can get high rank and SERP rank also for your site.