How can I design my own custom armor or weapons?

Use Tynker's Item Editor to change how your Sword or Armor looks.

https://www.tynker.com/minecraft/items/

Find the item you'd like to change, then use Tynker's editor to change how it looks! Here's a diamond sword that's now colorful and cool!

Save your work when you're done. You will download your work as an .mcaddon file. Just remember where you save your file. Open up the file using Minecraft (double-clicking it is easiest). This add-on file is also called a Resource Pack, and it contains the artwork you just created.

Create a new world in Minecraft, then enable the Resource Pack you just created. From the Create World screen, go to Resource Pack, My Packs, then select your item. It will then appear in your Active Resource Packs, like so:

Don't have your item in your inventory yet? You can enable Creative Mode to give yourself ALL possible items right away to see your new, cool, custom item! Switch modes by using the /gamemode creative command in chat.