The previous link has some good info, but I think it applies if you are already trading live. The reasoning behind starting with IWM/SPY is that you can minimize your risk of loss during your transition from paper to live trader.
As an example:
I am learning to trade the ES futures. I have developed a system that seems promising, am in the process of paper trading it and will eventually go live to test it. However, the ES is $50 a point and even though my account is adequately funded, I know I'll make mistakes once I switch to real cash. So instead, I'll be trading the SSO/SDS ETFs combination as a proxy to start (IRA account so I can't directly short) and my size with be something really small (like 10-25 shares) while I work all the bugs out.
As I hit certain milestones, I'll increase size and eventually migrate to 1 contract of the ES. If these milestones are hit quickly, then great. If not, I obviously had some issues that would have been really costly to learn with the ES.
My $0.0002 (would be $0.02 but I'm starting small)
As an example:
I am learning to trade the ES futures. I have developed a system that seems promising, am in the process of paper trading it and will eventually go live to test it. However, the ES is $50 a point and even though my account is adequately funded, I know I'll make mistakes once I switch to real cash. So instead, I'll be trading the SSO/SDS ETFs combination as a proxy to start (IRA account so I can't directly short) and my size with be something really small (like 10-25 shares) while I work all the bugs out.
As I hit certain milestones, I'll increase size and eventually migrate to 1 contract of the ES. If these milestones are hit quickly, then great. If not, I obviously had some issues that would have been really costly to learn with the ES.
My $0.0002 (would be $0.02 but I'm starting small)