To hide all retweets in your timeline, long-press the home timeline tab (bird icon) and select "Hide Retweets". (This is a filter, so it will be reset when you exit and re-enter the app).
To hide retweets from an individual user, go to that user's profile, select Options, select Turn off Retweets. This is a Twitter account-level function and does not apply to tweets already received in your timeline, but will impact your timeline going forward.
Hide Retweets is also an option in most other timelines as well (individual user's tweets, search results, lists) by long pressing the active tab and selecting Hide Retweets from the Timeline Options.