Download it at metamask.io
For Google Chrome download the extension: direct link
For Firefox download the extension: direct link
After the download just follow these steps:
First screen. Just click Accept
Scrow down the bar and click Accept
Enter a new password. Confirm it and click CREATE
MetaMask will display a seed to restore this account.
If you want to use this specific MetaMask wallet on another browser or computer you can enter the seed and use your account there.
This step is very important. Write down your seed on a paper and store it in a safe place.
MetaMask gives you the option of saving the seed on a text file. This is not recommended.
Click on I'VE COPIED IT SOMEWHERE SAFE
There you go. Your MetaMask wallet is ready to be used :)