Every US citizen must have health insurance starting 1/1/14 under the Affordable Care Act.
You can buy it privately or through Covered CA (coveredca.com). There are some exceptions to the mandate, but if you don’t qualify and you don’t have insurance you will be charged a penalty on your
tax return.

This year insurance companies and employers are filing form 1095-B and 1095-C with the IRS so that they can monitor and check that you had insurance coverage.  If you do not have coverage or even part year coverage, and you did not calculate the penalty on your return, it is very likely you will be notified by the IRS of additional tax/penalty due.  Be sure to let your tax preparer know about your insurance coverage and give them the 1095 forms with your tax documents when they are preparing your return.