There are a number of ways to create a new topic. The best way is to navigate to the type you want to create, either via search or browse, and click on the 'Add a new...' button in the upper left to add a new topic of that type. Autocomplete will assist you in seeing if the topic you are creating already exists. Once you have named your topic, click the link to view it and add more information to it.

Another common way to add a new topic is when viewing a list in Freebase. For example, if you are viewing a musical artist topic with a list of albums, you might notice that particular album is missing from the list. Click on the property title button for the list and you'll be able to add a new topic to it, in this case of type album. If the album you're adding doesn't exist, select 'Create New...' in autocomplete. The interface makes it easy to enter multiple items into a list as you can enter a value, click enter, and then hit enter again to open up another input box.