Ok I have two questions : (1). I'm forging a weapon and I don't know what..

On your first question:
Weight is verrrry important at the beginning of the game! If the weapon is really light, that means that it will be easier to double an opponent (attack twice) with a weaker character.
On you second question:
If you have killed him...Shame on you. Once you get Stefan, you may be tempted to overuse him and ditch Zihark, but don't do that. If you give Zihark bonus exp and give him mooch kills, he becomes one of the top 5 units in my opinion.

Yah, Zihark is worth getting because he turned out to be in my top 3.

1) ill give you the basic formula for weight.
if weight of weapon=strength of character, nothing happens. if weight>strength, then you subtract the remaining weight from your speed. so like this-> say mia has a strength of 14, and a speed of 20. she has a sword with a weight of 17. when she attacks her speed will go down by 2, because 17-14=2 (because she is not strong enough to wield that weapon) 20-2=18. her final speed in that battle will be 18. this is usually not a problem later on in the game, but early on it can affect a lot of characters trying to use steel weapons. thats all there is to it. (some people will try and tell you that it you actually lowers hit percentage, but thats not true... trust me. ALSO this DOES NOT affect your double hit rate at all. in order to hit twice your speed has to be 4 points higher than your enemy's speed.) when forging a weapon, always max Might(mt) and max hit rate as high as you care. set hit rate as high as you want to spend. dont touch crit rate at all. its not worth the thousands of gold you'll spend on it. and usually dont touch weight unless you've got someone with a low strength in mind, it really doesnt matter later in the game. on to your next question.

2) Zihark is the freaking bomb. use him. cherish him. enjoy him. give him an occult scroll, he'll learn astra-> 5 hit rape move, and he will practically be as useful as ike.