1. Give a proper tag for your question, example:"cryptography".
2. Have you tried googling for Vigenere cipher
3. Assuming you have googled the question, the top most result of the page should be this page
]. Have you read it?
4. Assuming you have read this
] and still doesn't know how to solve the question, here
]'s the part you should read.
It's your credit, do it yourself.
Looks like I've got too much free time on my hand, here
]'s a step by step instruction to solve it (Read the Series Information what you should read a series of 5 post starting from this post).