Get BitcoinThe easiest way I've found is using Coinbase ($10 free with this link).
Trading PlatformHead over to Bitfinex and create an account. Go to the Deposit Page and create a new address and send over your desired trading amount by going to your Coinbase account you just created and bought bitcoins on you need to send them to the address that you generated on Bitfinex. You do this by going to Accounts and then clicking the Send button under the wallet of the currency you purchased.
Buy IOTAGo to the the IOTA/BTC trading page. Using the Order Form on the left, buy as many as you want when your BTC is available.
I don not recommend you store your IOTA on an exchange so you can find a desktop wallet here.