You should read Using Indexes before implement the following code.

// Create Topic Name Index
TopicNamesIndex nameIndex = 
  (TopicNamesIndex) tm.getHelperObject(org.tmapi.index.core.TopicNamesIndex.class);

String needle = "tmapi";

Collection tmNames = nameIndex.getTopicNamesByValue(needle);

Iterator tmNamesIt = tmNames.iterator();

// iterate the results
while (tmNamesIt.hasNext()) {

  TopicName currentTopicName = tmNamesIt.next();

  Topic currentTopic = currentTopicName.getTopic();

  System.out.println("Topic Id: " + currentTopic.getObjectId());
}