HI, i am new in never winter, jz wondering what mean hit die?? like when i choose the class, it show hit die: d12, what mean tat?? what mean d12?? AND I saw the weapon there show 1d13 what it mean too??

1D12 means you roll 1 , 12 sided dice (result being anything between 1 & 12)
2D4 means you roll 2 , 4 sided dice (result being between 2 & 8)
a weapon that does +1D3 fire damage means you roll 1 , 3 sided dice & it could do +1, +2, or +3 fire damage.

When I say you roll, I mean the computer does that for you.

so izzit it mean, the base attack you own have, plus 1 or 2 or 3 fire damage?? and if 2d4 and i roll out 2d3 it mean i possible get base attack and +6 damage right??

You can go to www.gamefaqs.com and search for Neverwinter Night FAQs.
DSimpson wrote a 3rd Edition D&D Ruleset which explain the details.
He also wrote 2nd Edition reulset explanation, you may find it useful if you play icewind dale / torment, etc.

That is how the dice work.

Keep in mind that your "base attack" does not add to your damage, however. It only determines the number of attacks you make per round, and your accuracy with those attacks. If you want to increase your damage, you will need to use feats, magic items, or increase your strength score.

Keep in mind that when you make an attack there are two key rolls that are made- a roll to hit, and if that's successful then a damage roll. The roll to hit is 1d20 + your attack bonus (which is then compared against your opponent's armor class to determine if the attack hits), while the damage roll is whatever your weapon's damage is (e.g. 1d12, 2d4, etc) plus any damage bonuses (Str bonus, enchantment bonus, etc).