Which file serves as the main loading page?

I am having problems getting my host to properly point my domain to my blog. One thing I’m not clear on is which file is supposed to serve as the main loading page like as an equivalent to what index.html would be on a standard website? Is there also a way that I can see what a given php file would look like when loaded? Index.php for example. How can I tell what that is? All of this leads to what file do I need to have my domain pointing to in order for my blog to properly load?
Thanks