How do I open a pages document on m windows computer

On your Mac, you need to first export it as a Word Document. MS Word in Windows will not natively open a Pages document.

iWork is compatible with Office to the extent that iWork apps can read and write Office formats. But by default they save in iWork formats which are not compatible with Windows programs. You need to export the file - look at the menubar for sharing.

As has been noted, you need to save a copy as a Word document file for it to be opened under Windows since there's not a Pages native file format compatible solution on Windows.

If you need true compatibility on OSX and Windows you'll need to get Microsoft Office for the Mac. As you might expect, Word 2011 saves in *.DOCX format by default (same as Word 2010 or 2007 on Windows). Word "compatibility" is always somewhat hit and miss with non-Microsoft products (minor formatting differences crop up) so if you need as close as you can get to full interchangability, Word 2011 is it. I say as close as possible because even on Windows I've run into some quirks with different printer drivers in place or different versions of Word in play. Not huge differences, but line/page breaks have changed on me.

One intermediate solution would be Libre Office, the cross platform open source Office substitute you see used a lot on Linux systems (it's effectively the currently being developed successor to OpenOffice.org after there was a falling out over some things Sun did when turning it loose). It can be set to save to DOC format by default (unlike Pages--at least easily set to do it by default--I do recall there was some digging that used to be able to turn it on in Pages) but it's not the product's true native format. The major advantage is that it's free, but it has a definitely "non-Mac" feel to it.