How i can make a magic weapon?

I want to make a magic weapon, but how?

Are you talking about in game? Or a mod....?

If the former you need in the game to have acquired the magic weapon maker (either via being a loaded artifacts player level 2 or by monster dropping it..)..100 adren....and a..well, a weapon...

Then press use artifact (U by default..[ and ] to scroll artifacts...) and the random magic weapon is created....

A mod

There's no magic bullet, no quick and easy answer I can think of. I would suggest doing a lot of reading:

Druid's post about how to modify a mod

The Beyond Unreal Wiki is a valuable reference, but I would suggest asking Google specific questions (IE, "unrealscript modify weapon") and prepare to end up and one of the many BU Wiki pages.