Adding more domains to existing WP installation

I have read most everything I can find on the subject, but clearly I am the dumbest person to ever try this, so I REALLY need an idiot’s guide 🙁
I have a website (Let’s call it http://www.site1.com) running on WP, which I’m really happy with.
The problem comes when I try to add another site (www.site2.com)
Both sites are hosted by the same company, but I can’t figure out how to edit and publish the second (or third) in WP. Do I need another installation of WP, or can I use the same one?
I know this sounds really dumb, but can it really be that difficult to do?? 🙁
I’ve read up on multisite, Bitnami, networks and God knows what, but none of it makes sense.
Can anyone suggest an EASY way for me to have each website available in WP, on it’s own, with no complications??
Please help. This is really driving me mad!