if your talking about replacing the free hosting pop-under ads, this is the wrong place and this isnt an option on free hosting.  If it is for an account on paid hosting here is a little help:

I would use http://www.clicksor.com/ it gives you multiple options banner ads, popups…

You can also use google's system but it doesnt give you the extra options.

Both are very nice and easy to use. I like www.clicksor.com  a little more then google