The responses to your question remind me of my mother's reply when I asked, as a young wife, how to make dressing (as with chicken or turkey at Thanksgiving and/or Christmas). Turns out there was no recipe--other than a general agreement about ingredients--because all cooks make it to their own specifications or desires. But that wasn't helpful at to me then!
So I'll say this about borders: Look online and in a few books, if you have them, and if you don't, in a bookstore or the library. That's where I went when I first had your question. Experienced quilters, like some here, will give you concrete ideas and/or even "rules" for how borders are done. My borders, like my dressing, are now done according to my own "non-recipe", as in what I want and/or think looks good. But you have to do a few borders -- and make dressing a few times -- before you can trust yourself to make decisions like that.
Good luck!!