No offence to you in any regards.
I'm really confused by this question. Microsoft Internet Explorer is a browser. Yahoo is a portal. Both of these are completely different. You need not log on to microsoft internet explorer. Its already there on your computer and the only thing you need to do is 'start' it. and i'm assuming that you know how to do it.
Yahoo is a portal website. You can start microsoft internet explorer and then in the address bar type www.yahoo.com and go to Yahoo portal website.
if you are asking about changing the default home page. Then here is how you can do it.
open/start internet explorere.
click on tools
click on internet options
in the general tab, there should be a field that says 'home page'
type up any address you like, for example:www.yahoo.com
and click OK
by doing this, whenever you start IE, it'll directly go to that page first.
if you don't want any site to be there and just start IE blank. Then click on the box right below that text field that says "about:blank"
and then click ok
i hope it answers your question in some way.