Not sure if this is the right place or even the right website, but I have hopes to have a scene, either a battle scene or romance scene in rain. How can I do that? Do I just need a waterproof cover for the camera? How do I light it?

A custom rain cover or a splash bag makes things easier, but you can simply use a good quality plastic bag over the camera. Tape the edges to the sunshade or mattebox. Leave some ventilation at the bottom to avoid too much humidity buildup in the camera.

Rain reads best when backlit, particularly from two directions.

A battles scene, would of course, require enough rain to cover the field, so you are talking about real rain, and all the problems with working in it. Avoid it if possible.

For your smaller scenes, a lot can be accomplished with wet pavement, and a couple of rain hoses, one in the foreground, one in the background, and your talent then stands between them. They are actually not getting too wet, and you are looking through a curtain of rain. Additionally, there are some excellent rain fx in various programs. Boris I think has a good one, do a search.