you have to use the Twitter Rest API (documentation here: https://dev.twitter.com/docs/api/1.1) and make an HTTP call.
Fo example, in order to search tweets containing a particular keyword, you have to call an url like this https://api.twitter.com/1.1/search/tweets.json?q=keyword.
You can do this by using the HTTP Connector component, setting all appropriate parameters (also your credentials for twitter API) ad you'll receive data in JSON format.
For the first time, you could save the response from Twitter to a file using the "Universal Data Writer" component and then you can try to set the JSON reader component for parse it.
Hope this helps.