There are a un-countable blogs and forums where you can find hashtags for writers (e.g. Book Cover Cafe), so Google until your heart's content.
Here are my top six if you're desperate to get started:
#writerwednesday or #ww For networking with other writers - only for use on a Wednesday
#ff This means 'Follow Friday' - a great way to recommend and find other Tweeps you might be interested in following.
#fridayreads This is a weekly hashtag (used on Fridays, of course!) to show the world what you're reading at the moment. It can also be used to promote your book, if you can encourage your fans to include your Twitter name in their #fridayreads tweets!
#mustread Loved that book? Want to recommend it to others? Why not finish your tweet with this hashtag?
#amwriting This can be any kind of writing (e.g. song writing, letters, recipes). It's not novel or short story specific, which I can find frustrating, but it's interesting as a networking tool.
#writingprompt Great to get the creative juices flowing.
Let me know what you think in the comments box below, where you, as a writer, stop by Twitter. Have a great week - oh, and find me @MissWriteUK!