how to change a person look and form?

I want to put a custom stuff on roblox players like a custom backpack or a custom sword in side or back someone plz tell me

You can rig a custom character and put then name it "StarterCharacter" after that you put it in StarterPlayer --> StarterCharacterScript and when you will play it will show it like that.

After you can always make a script to define it better.