I met a guy called Bertan he asks me about Ilmater. Is it better to say you worship Ilmater or not? Does it effects the game in a wrong way? Well, I searched the forum and a little bit confused about halfling village, Captain Brage, Cursed Sword, Ulcaster, East of Naksel, Firewine Ruins, Skeletal Knights, Doom Slayer etc if I go these locations in a wrong order I may ruin the game I'M afraid some advices I seek, minor spoilers are welcome..

Can't help you with the Ilmater-guy, as I don't remember it in the game (it's been a while since I played), but there is no specific order in which you need to do those optional quests and locations you mention. One thing is, if you do some of them too soon, at a low level, you might end up dead, but that's all. Just do them when you feel ready for them, and you'll be OK. Only that cursed sword, you need to be in possession of certain items to get it which you won't have if you do those quests in a "logical" order, i.e. easier ones before harder ones. But since the sword is really terribly cursed, and utterly crap in combat, you don't need it ayway, so IMHO missing it isn't a problem at all.

Possible spoiler...perhaps...
*
*
*
*
*
*
*
*
*
*
*
*
Was Ilmater the apparition who appears at the Ulacster school ruins, and asks you to find a tome for him? I'm not 100% certain...

No, that was Ulcaster himself. The Ilmater priest shows up in the Northwest corner of the Firewine bridge map and tries to convert your party. I've never gone along with him, I've always told him I wasn't interested.