The first thing someone will see when they hit your Twitter feed is your picture, then your biography.Be sure your image is professional and matches your other online profiles. (LinkedIn, Facebook, etc.).
You need keywords in your biography that line up with terms applicable to what you do. Whether you're a farmer, or a SQL server administrator, you have to get that across.
You have 160 characters to fit in as many terms or keywords that apply to you and your skill set. The trick is keeping it from looking like a string of words.