what all should i need to knw before building a website?

Here's the basic checklist I use when I do freelance work:
1. Number of pages (how big/small of a website do they need)
2. Required applications (ie. blog, gallery, forum, shopping cart, contact form, etc.)
3. Targeted audience (ie. professionals, novice, etc.)
4. Timeline (ie. when is the first draft due, when is final product delivery)
5. Domain and hosting: do they already have a domain name? Do they already have hosting account? If yes, with whom, etc. Or is the project a website-only deal?
6. Amount of maintenance work needed after final product. If they are completely newbies, do you want to offer unlimited tech support or you prefer drafting a "user manual" so they take care of it themselves afterwards.
7. Graphics/Multimedia. Do they have ready-to-use material or do you have to find the material for them?
8. Payment. How are they going to pay you? You might want to draft a simple contract so that both you and them understand the payment schedule and services included.
9. Their competitors and/or their favorite website. That will tell you a lot about what you need to include on the website in case your client 'forgot to mention'.

Good luck!

Yeah, I agree with those mentioned in the previous comment, but I would also recommend CSS for design control.

oh man!! loads of things as in..
C,c++,java,html,corel etc
u have got to work hard..as one language is based on other..
good luck

I don't know if you already know how you are going to create the website, but if not, here are some suggestions:

Don't bother trying to code the entire website from ground up unless the client asks for something specific. Use a CMS like Drupal or Joomla. It makes your life much much easier, and in the end the result counts.

CMSes are highly customizable and if you know how to use them you can have a website up and going within a day.

The only additional things you might want to know if you use a CMS is how to setup a local AMP (Apache,MySQL,PHP) for testing. And how you transfer everything from your local machine onto the final server.

i understand that you mean what to ask the customer,
you need to ask everything you can think of..
like:
what colors they like
what they want it to look like - some like conservatie some like it fresh and wild.
what kind of music if there is one
do they want Flash / moving object or just show dry info
what they want to show and present and what not.
what they feel is the most important in their biz
what kind of people do biz with them - tatoo shop - golf shop
(i had a job for a restaurant that didn't want to advertise their wine list.)
so you'll get the best picture.
write everything in a contract and have them sign so later they will not demand sql shopping cart because they thought it was included.

HTML, how to use an FTP and possibly PHP if your website is going to have advanced features like order forms etcetera.