Step 1: Check to see if we are in an open enrollment period. If not, check to see if you have a qualifying event (if so, you can get coverage outside of open enrollment). If you do not have a qualifying event, you still have the option to purchase a short term plan.
Step 2: Determine if you qualify for a credit on the exchange. You will need to know your estimated adjusted gross income for the year. If your income is above the ones listed then you do not qualify for a subsidy. HealthCare.gov provides a chart.
Depending on your circumstances, you may be able to get a plan now that meets the standards of the Affordable Care Act. If you can, then you will not have a penalty. If your circumstances do not allow you to get a plan that meets the standards of the Affordable Care Act, you can still get a plan that is not ACA compliant. This means you will have a penalty, but you can still have coverage. These other plans are short term plans that are not guaranteed issue, and do not cover preexisting conditions or preventative care.
If you are a small business owner, you can purchase a group plan at any time. Even if you do not have employees, your state may have plans that allow business owners to purchase a group plan for themselves.
Generally speaking (in regard to cost only) you should go inside the exchange if you qualify for a credit or subsidy based on your income. You can use this calculator to see if you qualify for a credit. If you do not qualify for said credit or subsidy, there is no special benefit in going through the exchange.