For overhand slide, check out Dave Hole, a guy from Perth in Western Australia who plays overhand because of an injury to his pinky.

As for strings, I wouldn't go too heavy if you're just beginning, 10s would be fine.

And as for tuning, you'll be better off, and your guitar will thank you too, by dropping the tuning to say Open D (6th to 1st, DADF#AD) or Open G (DGDGBD)

For more slide info check out http://www.bigroadblues.com