dealing with rust

hi everyone. i recently purchased an old swb pajero. it is in surprisingly good shape, both inside and out. the only issue seems to be a smallish rust spot. the spot, about the size of a tennis ball, is in on the inside botom edge of the body. Only visible when I open the back door.

It doesn't seems like a problem now but I would like to fix it up so that it doesn't become one later on.

I would love some advise on how I could do it. I have never done this sort of hting before but would like to learn.

Thanks very much for your help