change the kind of metal
In general:
- A [METAL_PREF] tag in the entity entry results in the civ using the best (most expensive) metal they can find (ie, steel) for equipment
- A [MINOR_METAL] tag in the entity entry results in the civ using the worst (cheapest) metal they can find (ie, copper) for equipment
- [METAL_PREF:x] can be used to specify a metal for the civ to use if it's neither the most or least expensive (ie, [METAL_PREF:COPPER]). The civ will try to use that metal whenever it's got access to it.
- An [EQUIPMENT_IMPROVEMENTS] tag in the entity entry makes the civ use better-quality gear for their siege leaders
If neither the [METAL_PREF] nor the [MINOR_METAL] tags are in the entity entry, the civ will probably use iron (I'm not sure why this is, maybe the civ checks the next-best metal or something).
the weapons mostly used that siegers brings
You'll probably have to remove some of the [WEAPON:x] tags from the top of the entity entry - those tags determine which weapons the civ can make, so if you take everything but spears out the civ will only ever make spears, etc.
and if it was possible to make siegers wears mostly leather
Similar to the above - the quickest means is to remove all of the metal-related (chainmail, platemail, leggings, greaves, gauntlets, both types of boots, helms, caps) from the entity raw. Note that they'll still use metal shields, but taking the shields away will just allocate them a set of random shields and put you back where you started.
If you're not satisfied with having to severely limit your civs' equipment, you could always make some new equipment that can only be made out of leather and just give them that.
Also, about siegers using platemail - as far as I know the game will only let normal siegers appear with level 2 or lower equipment, so you could force the siegers to only use leather, but not force them to only use plate (without making new equipment that mimics plate but is really level 2, etc.).