Hi,
I would like to get started on Codecanyon and Envato.
Any guidlines how to do it faster. I know html, CSS, Javascript, a little PHP.
Do you think I could get some free code and modify it?

So you want to sell slightly modified code (from others) and make money from it?

Even free code often is licensed preventing you from doing this legally.

I do not mean to steal code. I mean using some code structure with different design.

Sure you can, plenty of code out there. Anything specific?

Just thought that someone who already sells on Envato will reply. I have no good ideas to get started.
I was thinking of creating a CSS website menu at the beginning and see how it works.

They're pretty strict on becoming an author and what you can sell on any Envato site. Anything you sell needs to be an original piece of work by yourself. You can be inspired by what someone else might have done but your code needs to be original. If it isn't you will be rejected by them (they review all submissions for selling) and it might also get you banned from selling on their site.

I would recommend reading more here
http://codecanyon.net/make_money/become_an_author

I believe that on envato sites (if you want to sell anything) they will make you take a test, and you have to pass it before you can sell anything.