There are tons of charitable groups that have organized to help people sign up with the new system, and the federal and state exchanges have people that will help you out on the phone or in person.
I live in Kentucky and our system is called "Kynect" so I can just use their website or just go to their downtown office to locate a "Kynector" that will help me sign up and is qualified to answer questions like yours. If you're concerned about this or anything else, finding someone to talk to either on the phone (1-800-318-2596 for the federal helpline) or in person is your best option.
Kentucky lets me pick whether I want a subsidy now or a tax credit in April, and I can tell them what I expect my income to be and get my subsidy based on that. I can also update my expected income throughout the year as many times as I want for any reason at all, whether it's job loss, a raise, or just because I want to. It may be different in your state.