When i am about to start playing BG2 for the first time (singleplayer), is better to start with one character, or i should start the game like multiplayer and premade all six characters?

Start with one character. You will miss out on NPC interaction and a lot of side quests if you crate a six PC party.

Even after completing the game a couple of times with only one created character and deciding to create a multi-player party, I only created 3 PC's. That way I was still able to add 1 or 2 NPC's as needed.

I personally don't think it would be much fun to play with a six PC party.

I started off with a multi game with four self-created characters and have included the various NPCs one or two at a time. But like Father Bronze says I discovered I missed out on a lot of the NPC interaction, even though I still ended up doing most of the side quests. At the same time tho' it's really neat to have more than 1 self generated character to develop, particularly if they're classes you can't find among the NPCs.

So I guess on balance I'd still recommend starting with a multi, because its the most flexible, you can balance the number of self + NPCs to your taste and can adjust that balance throughout the game as you meet new NPCs or want to import new characters.