Your question is slightly ambiguous… if you mean you want an alternative home page then the 2nd half of this blog post (http://blogs.alfresco.com/wp/developer/2011/11/01/advanced-share-customization-part-1/) describes how you can do that. If you want to change the Share so that each time you go a Site you land on the Document Library rather than the Site Dashboard then that may be more tricky. It is something that has come up before though and I know that it's something that we'd like to make easily configurable but there's nothing out-of-the-box to do this without some customization.
