Everyone talks about the weather or the front page headlines from the news. The more knowledge you possess and varied your interests, the more different subjects you will be interested in talking about.

Watch the world around you, and look for things that interest you. A broad vision of life will expand the subjects you will enjoy talking about.
Look for new friends in different walks of life or different backgrounds. Their perspectives, philosophies, and ideas may be interesting topics of conversation.
Listen to the people you talk with, and if they have unique interests, learn more about them. If you are frequently with someone who loves sports, take time to learn some of the basics of their favorites. The same would go for fishing, needlepoint, opera, or any host of potential subjects.
Search for new perspectives in your own interests, and keep current on the leading edge of discoveries in those areas. When Pluto was declared a non-planet some months ago, you can be sure it was a hot topic in the astronomical community, but people on the streets were talking about it too.
Study the reaction of people you normally talk with when you try to bring up new topics. If you see drooping eyelids and hear yawns, you are not stimulating them, you are sedating them, but if eyebrows rise, and you see response, you are on the right track.
Remember, with exception to offensive language or culturally taboo subjects, there are limitless possibilities. Movies, music, fashion, business, sports, politics, celebrities, school, and current events, even... the weather, offer unlimited conversational subjects, if you take a little time to be interested in them.
Develop hobbies and look for clubs or organizations that support them. This will be a unique ground to learn and share ideas and thoughts as your knowledge grows.

Watch at least a little local and world news whenever you can.
Read the headlines in various sections of the local paper.
Listen occasionally to talk radio, these are the experts on talking about a host of subjects.

When the group is talking about one subject, don't try to force a change in topic.
Spend time listening to others, for you will soon find few people willing to listen to you.
For older people, please do not go into minute details about your ailments. The only people interested in your medical history are you, your doctor and a lawyer, if you are suing for malpractice.
Repetitive talk is often seen in people obsessed not merely with their business interests, but with a particular mission; such as vegetarianism, deaddiction, faith healing etc. Introspect and find out if you are obsessed. This would make you far more acceptable, likable and respectable and enable you to further the true essence of your mission as well!