Hello there :) Welcome to Smogon!
I do want people to finish .savs for me, currently there aren't any that are needing done. If it says I do (I'll have to check on that) I'll have to edit it as I don't need any .savs done right now.
Credits, 1 per Gym Badge, and +1 for the Elite 4.
What credits are, are basically a "point" system. For instance, I can't trade you Pokemon to finish the game like I would if it was a Pokemon for Pokemon trade. So what credits are, is a way to give the person the chance to find Pokemon they want (from the shop) and use credits for that. Each Thread has a different value system for their Pokemon and credits. I always have 1 credit = 1 Pokemon.
Also, not to be rude or anything. I don't trust just anyone with the .savs I RNG. So it would be a good idea to get well engulfed within the community. Help out with things, make a trade thread of your own. Learn to RNG, things of that sort. :)