All houses can rekey locks to 1 key, and can be designed to reduce thieves bumping and picking. The first step is to check and see if all the locks are compatible to a common keyway. Try all the house keys in the different locks. Does one key fit into the others?
If one key fits into the other locks you are in fine shape. Pick the key you like the best (or have the most of) and have all the other knobs keyed to it. Remove the locks that need to be rekeyed and take them to a locksmith or a hardware store. Bring a key that works each of the locks and the key you want all the locks keyed to and give us five minutes a lock.
Some household locks, and most commercial, do not have screws on the inside trimplate (rose) making removal difficult. The trick is to locate a small hole or slot in the shaft of the inner knob. Using a miniature screwdriver or awl, push down on the keeper in the hole and it will release the knob from the lock. Once the knob is off, the trim plate will snap loose exposing the screws you need to undo to remove the lock.
All you need to take to the store is the side of the lock the key goes into. Leave the screws and other parts at home.
Another way to reduce the amount of keys you carry is to have your cottage keyed to the same key as your home.
What if one of your locks has a different keyway and none of the other keys fit in it? Bummer. You will need to purchase a new lock that shares the keyway of the locks you are keeping. Some stores will key a new lock to your existing key just for the purchase of the lock. Be sure to ask and save yourself the hassle of carrying an extra key on your ring.