Insulation is the invisible coat used to make your home energy efficient. Ideally you never have to see or be involved with your insulation and if your home is newer, this very well may be the case. But for older homes, the insulation could have fallen, gotten moved and displaced due to the rigors of day to day attic usage, or simple been done so long ago that it is inadequate to keep the house comfortable.
In any case this is the type of project that lends itself to doing it yourself, though it is fraught with perils such as having to spend large amounts of time in hot or cold attics, or crawling around underneath your house and putting up insulation above your face as it falls on you.
If you want to take on a project like this, you need to research the amount and type of insulation to install, calculate how much you will need, and get to work.
Another option is to do research on a site that includes all the information you will need. One such site, Central Virginia Insulation, does a very thorough analysis of these types of projects.