Does anyone know good jayce guide? Like youtube video?

http://www.mobafire.com/league-of-le...-videos-227352

^ that's a pretty good place to start. That's generally a pretty popular and pretty reliable guide and the vids were helpful when I was starting with Jayce...

You should also watch some streams every now and then and watch how some of the high level players use Jayce...you'll get a lot better understanding of how to use his abilities.

which stream is the best to watch?

Don't use mobafire. It is unreliable, at best, and outright terrible most of the time. If you need a place for guides look towards lolpro.com and solomid.net. They each have pro player's guides written by the player's themselves.

Here is a good guide, one that features both of the popular builds for Jayce and when to use them. It also has a replay section.
http://www.lolpro.com/guides/jayce/3...ane-crs-angush

As for streams, I don't know of a particular Jayce player that streams, but if you're lucky you might catch one of the higher elo players on Jayce, like Dyrus or Wingsofdeathx (at least on the North American side).

How about other players like in Europe or Asia etc.

Consider that season 3 is almost here and many changes are being made. One of the most relevant is a lot more AD caster items, and AD is about 15% cheaper.

This is a pretty big deal because Jayce has huge AD ratios. Ive always built him like a Riven or Pantheon, instead of using Triforce. i.e. Dorans blades, brutalizer, ionic boots, hexdrinker, giants belt>warmog, Aegis, bloodthirster,etc.

Triforce works in SR though because Jayce has enough abilities to trigger its damage proc often, and he likes the bonus speed boost, slow, and general utility. Plus the triforce is just so efficient that many many champs use it as their sole damage item, and pile on defensive items afterwards.

But now I expect Jayce to start using more AD caster items.