VIX (like other indices) is not directly tradable.
You cannot (directly) buy the VIX. You cannot sell the VIX.
Here you can see more detailed explanation on why you can't buy or sell a volatility index.
Ways to Buy VIX
However, you can buy VIX derivatives and other securities related to the VIX. They are especially:
It is not that simple though. The prices of these securities, although related to the VIX, do not move in the same way as the VIX index itself.
For example, you can go long a VIX futures contract and still lose money while the VIX index is rising. The pricing and trading of VIX derivatives (and volatility derivatives in general) is very complex and can involve high mathematics and statistics. Without any ambition to make you ready for profitable VIX trading, this page can serve as a good introduction: Trading the VIX.