This guide only works if you currently hold some bitcoin and you don't mind using some of it to wage on Ether.
Anyway.
1. Go to poloniex.com and create an account. This is a two minute job not like on a lot of exchanges. All you need to do is verify your email.
2. Under the balances tab click deposits and withdrawals then wait a few seconds for all the crypto currencies that can be bought and exchanged to load.
3. Click deposit on the BTC column and then click show me the deposit address.
4. Send some BTC to Poloniex. This should show in your account instantly.
5. Click the Exchange tab and ensure ETH is selected. Directly below the graph you can place a buy order for ether using your deposited BTC. Buy some ETH.
6. Goto
https://www.myetherwallet.com generate a new ethereum address. Download the JSON file and save your address. Print or store a copy of your private key for later.
7. From Poloniex send some ETH to your myetherwallet address.
That's it you now hold Ether. To view your holdings at anytime go to myetherwallet and click view wallet info. You will then be prompted to open your wallet file from your PC and use the password you chose for your wallet.