The easiest is probably through a stock broker. Futures are risky (typically highly levered, though less so for BTC) and they usually make you have a decent chunk of change before they'll give you access.
I have a few brokerage accounts at a few places (Fidelity, E-Trade, Vanguard). Haven't done much margin trading and never worked with futures (done a little with options) so I just don't know what to look for in the interfaces. Is there a "ticker symbol" that represents a future contract for Bitcoin? Are there multiple parties offering such contracts? Is there a broker that you know of who's currently offering these contracts?
Two futures exchanges run Bitcoin contracts, CME (trading under BTC) and CBOE (trading under XBT). They're not a lot different outside of size, CME's contract represents 5 bitcoins while CBOE's represents 1. Here's a decent listing of the differences:
It's definitely worth reading up on how futures work before trading them, but it's similar to stocks in terms of using a ticker[1] and making buy/sell orders. A key difference is margin, but with the volatility of bitcoin both exchanges have really high margin limits so you can't get into too much trouble. The other key difference is you're trading a specific month so you need to be more correct on timing than you would for a stock.
[1] The tickers work a little different than stocks in that the root is XBT or BTC, but the full symbol for the contract includes the month and year. This is similar to options. Your trading platform should make this clear.