how do i make a link so when i click on it a new window pops up?

if you just want a plain new window to open, you can use target="_blank" or target="new", like so:

<a href="link.html" target="_blank">Link text</a>

_blank will open every link in a new window, whereas new will open one window, and as long as that window is open, all the other links that are also targeted to new will open in that same window.

do note that the "target" attribute is deprecated in xhtml. i don't know why they are moving away from it but i guess they don't want you to open new windows

do note that the "target" attribute is deprecated in xhtml. i don't know why they are moving away from it but i guess they don't want you to open new windows
It depends on the type of XHTML you're using. I use Transitional 1.0 and it validates perfectly fine. I suppose Strict 1.0 or anything beyond that limits the rules a little bit (who really needs that much perfection anyways?).