Generally, very few modifications to your home are needed for installation.
Collectors will be added to your roof or mounted on the ground, a solar preheat tank will be installed near your water heater (or you can install a combined tank), and the plumbing to connect the two will be installed in your walls, crawl space, basement or attic.
Flat panel collectors weigh about 200-250 lbs. and residential scale evacuated tube systems weigh about 175 lbs. Most roofs can support this weight, but some may require additional support.
More serious considerations, from a structural perspective, are wind-induced loads and concentrated loads. Remember that the snow or ice that used to be spread out evenly over your roof will now be concentrated at the mounting feet. For these reasons, a structural analysis is often required by the building officials who review plans and issue permits for solar systems.
It is a good idea to install solar panels over a roof which has 15 of more years of life. Some people do a partial re-roof under the area of the solar array if they are not prepared to do a complete roof replacement.
Fire marshalls are now asserting their authority in the permit/plan review process to make sure they have room to work in the event of a fire in the home/building. A qualified solar contractor will help you work through all of these issues including working with a structural engineer.