Most firms will allow you to trade the ES but the level of service, tech platform and margins varies. I've been with Berkeley Futures for some time and can recommend them based on their personal service and low margins. Their currrent account minimum is 10k Sterling
If you want to start out with smaller you might want to consider Interactive Brokers, which start at 10k USD. In any case you shouldn't put real money on the line before you're at least 90% sure of what you're doing.