How to put minecraft on a usb?

Go to My Computer (or equivalent) then either Program Files or Program Files (x86). Then find the Minecraft folder (it might be in a folder which shares the name with whoever develops Minecraft). Then Copy it and Paste it onto the memory stick. Plus go to My Documents (or equivalent) and see if there's anything there that also needs copying.

You need to backup

(open 'computer' folder and follow the folders)
C:\Users\ Owner or profile name \AppData \Roaming \.minecraft

(appdata folder is hidden, you will have to type it in when your in your profile folder, add it on the end)

you can then put the file/folder in the same place it was when you reinstall and it should be fine.

or use this:

Savegame Manager
http://www.gamesave-manager.com/

it can be slow and take a while to detect and backup the save games, but it's pretty easy to use and backs up and restores all the files needed.

First you will have Minecraft anyway Because minecraft.net saves your acc For Ever All you need is to remember your e-mail to login in and your password.
You want to move your minecraft save files? Then Go in the start button (Where You Shut Down Your Computer Restart it e.t.c) And type in %appdata% Then go to the folder called.minecraft there should be a folder caled saves that folder copy and paste it into you USB

When You Wipe your computer install Minecraft from minecraft.net and then go again in %appdata% go.minecraft and then in the saves folder copy the saves files from your USB and paste them into the folder Saves then your worlds-saves should be there!

That's All Hope You Find It Usefull.

Minecraft saves your account. Means that to play the game you just need to download Minecraft on another computer and log in on your existing one. No need to copy past your Minecraft folder on another computer.
You should only do this if you want to keep your mods, texturepacks and save files.