You are not required to have MobileMe subscription in order to be able to build a website in iWeb. There are three ways to publish your future iWeb site: MobileMe, FTP and local folder on your desktop. You can use the last two options to upload the website. FTP can be a tad complicated for a not-so-savvy person since you have to have server address, user name and password handy. All of that can be obtained from your Yahoo control panel or the how-to/faq section. Anyway, good luck and let us know how it all works out.
Can I use Iweb to upload the page made on Mac to Yahoo? < Absolutely, you just have to get the correct upload details for yahoos servers then go into the site name bit on iweb and you'll get Publishing settings ... choose FTP and put in the correct elements and it'll publish anywhere.
If I sign up for MObile me, will I get hosting for web page? < Yes. You get web hosting included with MobileMe - you can also use a custom domain to host it against as in www.mycoolsite.com ... it has limits... no php or mysql support for one but thats about it.
I've hosted my site using mobileme since launch .... i use rapidweaver for my site though rather than iweb but thats just me.