The emini futures are a futures market. Have you trader futures before? Open a brokerage account (Interactive Brokers are popular). Put up a 'performance bond' (aka margin deposit), learn to use the execution software (what allows you to buy and sell), and away you go.
The Chicago Mercantile Exchange website is a good place to read up on things like how much performance bond is required (your broker will explain this even more easily), the hours the market trades, what the futures contract specifications are (what is it you are buying and selling), things like that.
There is really not that much to it, but it can look confusing at first. Where you coming from, what have you traded, how, when, etc....?