If you're buying insurance on your own, you've got to shop around for the best price. You can shop the federal health exchange created under the Affordable Care Act, or find your state exchange, by going to Healthcare.gov. Most states have various plan options available at different prices. What's more, the federal government offers subsidies to people with income below a certain level.
Insurance companies also sell plans directly to consumers outside of the federal exchange, but the plans are pretty similar and you won't receive a subsidy.
Since the passage of the Affordable Care Act, insurance companies can no longer discriminate against you because you have preexisting conditions, but you will likely still pay more if you are older, and plan pricing varies widely from state to state.