Identify your concerns. If you are worried about vandals, then more lighting and security cameras on the outside of the house may be needed. If you have valuables in your basement, you might consider a motion sensor there plus sensors on the doors that lead to it. Once you know what you want to protect, you can better assess your needs.
Examine all your doors and windows. See which methods of entry you think are the most susceptible to compromise and determine if you want sensors on all of them or if you are only concerned about the lower level or the garage.
Analyze cost. It might be nice to have security cameras and motion sensors in every room of the house but that would probably not be cost-effective. Prioritize your needs and figure out what you'd be willing to spend the most money on.
Request a consultation. Most security companies will provide a free walk-through of your house and offer an assessment of what your risks are. Once you have your own thoughts on what you need, comparing them to those of an expert can only benefit you. From there, you can start comparison shopping once you have a general system requirement in mind.