well in that case on your xcom foder go to standart/xcom1 you have several .rul files , install notepadd++ open the itens.rul and you can see the basics stats for all items in the vanila xcom. after that acquaintance you will have to study extrasrtings.rul for naming and extrasprites.rul for images
after all that just download a simple weapon mod i see how those 3 .rul files exist and link to each other
Amor is a bit more complex but check armor.rul after all those and you almost there
tips: notepad++ language i go for python , also start with simple item and after that go for armor