I really recommend you to use the subway, otherwise you are going to have to look for a parking spot on the streets (streets close to Boston common are usually the best place to find parking spots) or you can also park at the Boston common garage. Either way you will need to walk a little.
There is "secret" free visitor parking in Beacon Hill, otherwise, like Cristian said, you can find parking but you WILL have to walk a little! There is metered parking in Beacon Hill and Back Bay that's still WAY cheaper than the Boston Common Garage.
Thanks! Your answer is awaiting moderation.