You'll need an EOS wallet in order to purchase POW. For Chrome users, we recommend LeafWallet – a light and easy-to-use browser extension. You can add it by clicking on Install EOS wallet.
Alternatively, choose any of the other reliable EOS wallets and services, including MathWallet (multiplatform), NOVA (Android, iOS), MeetOne (Android, iOS), TokenPocket, and Scatter.
For all wallets, you'll need to enter your EOS account name and your keys file. Remember that your EOS aren't stored in a wallet, but rather on the blockchain. So you can access them from any wallet using your account name.