how to choose Keywords for Website?

I found a thread on this forum, it can help you

http://www.webmastersun.com/threads/2203-How-to-Find-the-Best-Keywords-For-Your-Website

In other words, which niche do you want to choose keywords for?

and which type of keywords you will choose, short or long tail keywords?

To do some basic keyword research you should checkout Google Keyword Planner and Google Trends.
Some paid sites that can help you would be KWFinder, BuzzSumo, and Moz. These are just my fav, there are a lot more tools out there to help you.

To find keywords, You can use several tools like Long Tail Pro, WordTracker etc etc. BUT I personally recommends Google Adwords Keyword Planner.
It is really a great tool and It will allow you to find profitable keywords.
If you want to choose best keywords for your niche. Then find some blogs related to your blog and then try to find that what keywords your competitors are using so as to gain idea about choosing keywords.

When you are going to choose a keyword
Focus on Good Phrases
Avoiding "Vanity" Keywords
Using Google's Wonder Wheel
The Value of Repetition

Keywords should be chosen with the subject of your website. You can use tools like Google keyword planner to get good suggestions for your keywords with the number of searches. It is preferable to chose the keyword priority based on the number of searches they have in Google. Short keywords are the most beneficial to rank your website and gain popularity in search engines. Apart from them you can also get suggestions and choose long tail keywords with less number of searches to help rank your website faster with them. They are helpful to build some part of the website traffic.

If you want to build a website, first think about what do you like, and go for it.

Go to Google Keyword Planner, enter your keyword, select your targeted location from the options.

Check how many searches the keyword has.
TIP: choose 2-3 big keywords, and 100+ secondary long tail keywords.

Next install:
mozbar
seoquake
chrome or firefox extensions/addons.

Go to Google.com or your countries Google. Enter your keyword, but first make sure you have those two extensions/addons activated in your browser.

You want to see what are the top 10 results, how powerful are the domains and pages of those results.

If PA (Page Authority, according to moz/mozbar is lower than 40) you can go for that keyword. Do all of this with all your keywords, but first start with your 3 main keywords.

Also check how many backlinks a page has with seoquake, the age of the domain. and so on.

Hope this helps.