Decide between a group or a private tour. On a private tour you might have a different guide in each new destination, whereas on a group tour you will have the same guide or tour leader throughout. Traveling or visiting the sights with a group is fine if you are interested in meeting other people and don't mind moving at the pace of the group. A private tour is much more flexible, allowing extra time to ask questions or take an interesting detour.
Think about whether a general or a specialized tour is best for you. If you are visiting London and only want a general impression of the city and its history, then a regular sightseeing tour is ideal. If you have an interest in Harry Potter or haunted London, then a specialized tour will make the trip more memorable.
Consider how active you would like to be. There are many ways of seeing a country or a city; from cycling and walking tours to organized trips that fly you around the country and pick you up in private cars. If tours that shepherd you around by bus with scheduled stops seem too constricted, then look into walking or cycling tours. Choosing a tour with plenty of time for optional activities means you have the security of an organized tour and the flexibility to go off on your own.
Look for a tour company that meets your expectations in terms of comfort and professionalism. With big international tour companies such as Gap Adventures (gapadventures.com), you can expect to be well looked after and have everything planned to the last detail. Choosing a local tour company, though, can give you greater insights into the place you are visiting, more flexibility and more affordable private tours.